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6/20/2014 10:31 P.M. ET

Rondon's health priority for cautious Cubs

CHICAGO -- Righty Hector Rondon remains the Cubs' No. 1 closing option, but the club is being careful with his usage.

Rondon had experienced some tenderness in his elbow, so the Cubs reduced his workload. After pitching in four straight games in mid-May, he's made just four appearances this month. Rondon pitched a scoreless inning of relief on Monday in Miami, his first game since June 6, and hasn't pitched since.

Other members of the bullpen have filled in admirably for Rondon. Since Rondon's last save on June 4, Pedro Strop, Neil Ramirez (three times) and James Russell have all picked up saves.

Given the success of multiple relievers in the role and the need to closely monitor Rondon's elbow, manager Rick Renteria plans to stick with the closer-by-committee approach for now.

"We'll continue to just be mindful of how they are feeling physically and go from there," Renteria said. "No timetable as to how we're going to do it, but we'll just be very mindful and use them as we need."

Epstein reflects on Bryant, Triple-A promotions

CHICAGO -- The big league club has been playing well lately, but with the Cubs still in rebuilding mode, the focus for many fans turns to the potential future stars moving through the system.

A number of the Cubs' most highly regarded prospects were recently promoted a level, most notably No. 2 prospect Kris Bryant, who hit an opposite-field two-run homer Thursday in his first start for Triple-A Iowa. Bryant tore up Southern League competition, posting a .355/.458/.702 slash line with 22 homers and 58 RBIs in 68 games for Double-A Tennessee. As president of baseball operations Theo Epstein put it, it was time to challenge Bryant with something new.

"We're proud of Kris. He's had an unbelievable start to his first full professional season, and he had accomplished a lot of the developmental goals we set out for him at Double-A," Epstein said. "We felt like a promotion to Triple-A to continue to challenge him would be good for him in the long run.

"He still has some things to work on. He'll continue to work on them at Triple-A, but seeing some more advanced pitching, some guys that can execute on a more consistent basis, try to find his holes, change speeds -- it'll be good for him. It's the next step in his development."

Should Bryant continue to excel, the next stop is the big leagues. But Epstein reiterated the Cubs don't plan to call him up this year.

"I don't think it's the right thing to do in someone's first full professional season, barring extraordinary circumstances both to the player and what's going on with the big league club," Epstein said. "I think it would be a tremendous accomplishment to dominate Double-A the way he did the first half of the season and go to Triple-A and continue his development, stay healthy all year and be productive all year and continue to work on his weaknesses."

Relievers Arodys Vizcaino and Armando Rivero were also promoted to Iowa.

Vizcaino threw 17 1/3 innings for the Braves in 2011 and was a key piece in the Paul Maholm deal that summer. He missed all of 2012 due to Tommy John surgery and all of '13 due to calcium buildup in his elbow, but the 23-year-old seems to have regained his stuff. The righty, who has been consistently reaching 98 mph on the radar gun, posted a 1.99 ERA with 26 strikeouts in 22 2/3 innings at Class A Advanced Daytona and Double-A Tennessee this season.

"With Vizcaino, it's really just healthy and continuing to be consistent with his mechanics and throw that breaking ball. I think he's been kind of using it more and more lately and getting a lot of swings and misses with it," Epstein said. "So using all his pitches, getting in different game situations -- he's a big league pitcher, he's been up before, it's really just a matter of sort of getting back in the routine, feeling comfortable after the surgery, and then he'll be up here."

Rivero was impressive at Tennessee as well this season. He posted a 1.56 ERA and a 0.98 WHIP in 34 2/3 innings, while striking out a whopping 54 batters in 34 2/3 innings.

"I think with Rivero, it's really fastball command. He's got really good stuff, and when he's locating his fastball, consistently getting ahead and finishing guys off with his slider, it's really impressive punching out a ton of guys," Epstein said. "The better hitters there will force him to be more locked in and continue to work on that fastball command."

Cubs manager Rick Renteria said he's well aware of what Bryant and other impressive prospects are doing.

"But I am more concerned with the guys that are here right now. I have to be," he said. "I think the biggest thing that we watch for is I have to really concern my focus on making sure I'm reading these guys well and understanding what's going on with them. On the periphery, obviously we're aware of everything that's going on."

Cubs happy with Schwarber's quick progress

CHICAGO -- Kyle Schwarber, selected by the Cubs with the No. 4 overall pick in the 2014 First-Year Player Draft, was promoted to Class A Kane County on Tuesday after just five games with short-season Boise.

Cubs president of baseball operations Theo Epstein explained that the organization had planned to send Schwarber to Boise for about a week to "get his feet wet," and rather than have him take a nine-hour bus ride to Eugene, Ore., for Boise's road trip, it made the most sense to send him to Kane County.

"We're really happy with the quick adjustments he's made to pro ball," Epstein said. "The on-field stuff takes care of itself, but sort of how he's handled things, mentally. He's been through a lot this last month, and he's been real consistent, steady and off to a great start."

Schwarber posted a slash line of .565/.593/1.217 with four homers and 11 RBIs with Boise. Dating to his college days at Indiana University, hitting has never been the issue for Schwarber. What remains to be seen is what position Schwarber sticks at.

Schwarber was a full-time catcher at Indiana, but there is some question as to whether he's good enough to do it at the Major League level. Epstein said Schwarber will catch once or twice per week to keep that area of his game fresh, and he'll play left field the rest of the time.

"With Kyle, I talked to him about it, and we decided for this year, his first exposure to pro ball, we're going to have him see a lot of time in left field. ... Then we're going to sit down at the end of the Minor League season to see if it's the appropriate time to make a call," Epstein said. "If we decide that he looked [good] in left field, and his development path with his bat might be so fast that there's maybe not going to be time for his defense behind the plate to catch up, then we can go a different direction, and maybe he can even be a candidate for the Fall League."

Castillo hits single in first Minor League rehab game

DES MOINES, Iowa -- Cubs catcher Welington Castillo went 1-for-5, hitting a single in the sixth, in his first Minor League rehab game with Triple-A Iowa on Friday night.

Castillo grounded out in his four other at-bats.

His rehab was delayed one day because of wet grounds at Principal Park. Castillo was in Thursday's lineup but rain delayed the start of the game for more than one hour. Iowa manager Marty Pevey said he scratched Castillo as a precaution because he didn't want to risk having the catcher slip on the wet area behind home plate.

Castillo has been on the disabled list since June 2 with left rib cage inflammation. He did take batting practice on Thursday and caught in the bullpen prior to the game. Castillo was expected to play Saturday.

Extra bases

• The Cubs just completed a stretch in which they played 20 of 26 games on the road. Friday's opener with the Pirates began a 10-game homestand.

"I thought the guys did really well. We ground it out," Renteria said of the stretch of games. "They keep playing the game, and I don't think they were thinking that it was a 20-game stretch, I think they were just more focused on playing the game every single day."

• Chicago is 10-7 in June entering Friday's game. The 10 wins in the month are tied for second among National League clubs.

• The bullpen's 3.08 season ERA is the fourth lowest in the NL, and is more than a full run lower than its 4.15 ERA through 70 games last year.

Carrie Muskat is a reporter for MLB.com. She writes a blog, Muskat Ramblings, and you can follow her on Twitter @CarrieMuskat. Joe Popely is an associate reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.