PHOENIX -- It appears Cubs pitcher Randy Wells will get another crack at filling Ryan Dempster's spot in the rotation when it comes around again Tuesday against the Mets.

"We'll stick with Wells for right now," Cubs manager Dale Sveum said. "Something could change, but right now Wells will pitch on Tuesday."

The 29-year-old right-hander struggled in his first opportunity, surrendering five hits, four walks and three earned runs over 3 2/3 innings in a 7-0 loss to the White Sox on Wednesday.

In 25 2/3 innings this season, Wells has walked 20 and struck out only 11.

"The walks have killed him, especially the non-competitive ones," Sveum said. "He's got to get the ball over the strike zone. We have to get that turned around. You have to make them put the ball in play."

Rizzo promotion to Cubs imminent, but when?

PHOENIX -- Cubs manager Dale Sveum isn't hiding the fact that first baseman Anthony Rizzo's promotion from Triple-A Iowa is coming soon, he just doesn't know when exactly it will be.

One thing is for certain though, Rizzo, ranked by MLB.com as the No. 29 prospect in all of baseball, won't make his Major League debut in Arizona this weekend.

"Not in the next few days I know that, not here," Sveum said. "But like I said, it's obviously coming soon."

In 66 games with Iowa this season, Rizzo is batting .350 with 59 RBIs and a Pacific Coast League-leading 23 homers.

After the Cubs finish their three-game weekend set with the D-backs, they'll return home Monday to face the Mets.

"I don't know if there's a date on it," Sveum said when asked if the promotion will come during the upcoming homestand. "We just don't have a date yet."

Barney playing flawless defense at keystone

PHOENIX -- Since April 17, Cubs second baseman Darwin Barney has faced nearly 300 chances to make an error. But each and every time a ball has come his way, the 26-year-old's defense has been spotless.

Entering Friday night's series opener with the D-backs, Barney was riding a 56-game errorless streak, spanning 462 1/3 innings.

"He's done an unbelievable job at second base," Cubs manager Dale Sveum said. "He's got good range, especially to his left. He's doing a great job. He's been far and above anything that I saw from the other side of the fence."

The last Cubs middle infielder to put together a streak longer than Barney's was Mike Fontenot, who had a 90-game errorless run from May 7, 2008-June 30, 2009.

"When you get to 50 games, that's pretty impressive, it doesn't happen too often with middle infielders," Sveum said. "When you're playing second base and you have to turn double plays, take relay throws and all that stuff, that's where some of the errors come in."

In addition to Barney's defensive prowess, Sveum believes the infielder is just a few mechanical tweaks away from being as successful at the plate. In 238 at-bats this season, Barney is batting .265 with 15 doubles and 22 RBIs.

"In the past, he's tried to jam the ball into right field," Sveum said. "But he's starting to come around and know he can drive the ball and do more things with the bat than hit singles to the right side."

Worth noting

• Dempster, who was placed on the 15-day disabled list Monday with right lat tightness, traveled with the club to Arizona for what was essentially a three-game road trip after the Cubs' midweek series on the South Side against the White Sox.

• Cubs third baseman Ian Stewart, currently on the DL with soreness in his left wrist, will continue his rehab Saturday by swinging a bat at Chase Field.

"He'll do some swinging tomorrow, nothing live though, just swinging the bat," Sveum said.

• With the Cubs flying to Arizona a day early, the team received a day off Thursday and Sveum, whose family resides in Scottsdale, used the time away to watch his son, Rustin, play in a club baseball game Thursday.

Sveum said his son collected a leadoff double, but his team lost, 2-0. The manager didn't fully stop working while watching the game either: He came away thinking highly of the opposing team's pitcher.

"He was throwing 85-87 [mph], pretty impressive," Sveum said. "I should've got his name."

The Cubs drafted Rustin in the 39th round of the First-Year Player Draft.