CHICAGO -- Randy Wells needs his sinker to behave if he is going to capture the National League Rookie of the Year Award.

Wells, who will take the mound Tuesday against the Astros, has a 9-7 record despite a 3.06 ERA. He has to be considered a top contender for the honor, along with fellow starters J.A. Happ of Philadelphia (10-3, 2.63) and Tommy Hanson of Atlanta (9-3, 3.15). But he also must correct a mechanical problem that rendered his key pitch ineffective in his previous start, Thursday against Washington.

The sinking fastball serves as Wells' go-to offering, and as a result, he is in the top 20 in the National League in the ratio of ground balls to fly balls. Against the Nationals, Wells could not get the pitch to work and instead was forced to go almost exclusively to his offspeed stuff. He gave up five runs in 6 1/3 innings, including three in the first.

"I don't think it's anything with the actual pitch," Wells said. "I just think it's a little mechanical issue. My hands weren't really moving in sync with my body and it was causing me to fly open."

That made Wells leave his pitches off the third base side of the plate, out of the strike zone.

Wells worked to correct the issue during a side session between starts and during a drill Monday, in which he repeated his delivery over and over again with a towel in his hand instead of a ball.

"It just makes you really work on your mechanics and fine tune them," Wells said.

There was also a mental component to Wells' struggles Thursday. In addition to keeping his front shoulder closed, Wells said he has to trust his sinker, rather than consciously try to give it more movement.

He also intends to make better adjustments in between innings if necessary.

"I don't want to say it was a rookie mistake, but I let the game get out of hand," Wells said of his previous start. "I let the emotions take over. My heart rate started getting up and I was going a mile a minute, and I wasn't concentrating on what I had to do."

Despite his success this season, Wells said he still does feel like a rookie.

"There's something to learn every day, and I'm learning that lesson," he said. "It's been exciting, and I'm definitely a little more comfortable, a little more confident going into a start than I was earlier. But at the same time, there's a lot of things to pick up, and you never take a day off."

Pitching matchup
CHC: RHP Randy Wells (9-7, 3.06 ERA)
Wells is trying to become the first Cubs rookie to reach double-digit wins since Kerry Wood won 13 games in 1998. Wells now has given up five earned runs in two of his last three starts. In two starts against Houston this year, he's not given up a single run over 14 innings.

HOU: RHP Brian Moehler (8-9, 5.26 ERA)
Moehler turned in a quality start in his last game, though he wasn't around to get the decision in a 4-3 victory over the Cardinals. He allowed three runs on six hits in six innings. Moehler has been far better on the road this season, compiling a 4.35 ERA compared to a 6.14 mark at Minute Maid Park.

Tidbits
When rosters expand Tuesday, the Cubs will recall first baseman-outfielder Micah Hoffpauir, infielder Andres Blanco and relief pitcher Justin Berg, according to manager Lou Piniella. Reliever David Patton will get the call the following day, and three more players will join them once Triple-A Iowa's season is finished. ... Piniella said left fielder Alfonso Soriano, who is sidelined with a sore left knee, might play Tuesday but is more likely to make his return Wednesday in the series finale against the Astros. ... Mitch Atkins stuck out eight and gave up five hits in eight scoreless innings, as Iowa beat Oklahoma City, 2-0, on Sunday. ... Tyler Colvin homered for Double-A Tennessee on Sunday to extend his hitting streak to 15 games.

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Up next
• Wednesday: Cubs (Ted Lilly, 9-8, 3.35) vs. Astros (TBD), 1:20 p.m. CT
• Thursday: Cubs (Ryan Dempster, 8-7, 4.15) vs. White Sox (Carlos Torres, 0-0, 6.75), 1:20 p.m. CT
• Friday: Cubs (Carlos Zambrano, 7-6, 3.91) at Mets (Bobby Parnell, 3-7, 5.81), 6:10 p.m. CT