Aardsma, 24, had not allowed a run in seven innings over five games for Triple-A Iowa and had two saves and 11 strikeouts.
"We want him to come here and throw the ball the way he's been throwing down there," Baker said.
Acquired from the Giants in May 2005 as part of the LaTroy Hawkins deal, Aardsma was 0-1 with a 2.89 ERA in 10 spring appearances this year. He pitched in 11 games for the Giants in 2004.
Bob Howry and Scott Eyre will still be the primary setup pitchers for closer Ryan Dempster.
Baker used 12 pitchers last year out of necessity rather than desire. He'd prefer 11 so he can have that extra man on the bench.
"Twelve gives us the best shot to win," he said.
Strong arm: Ask Ronny Cedeno about his throwing motion, and he starts to sound like a pitcher. The Cubs' young shortstop is working on keeping his right elbow up because when he drops it, the ball tends to sail on him. Cedeno has three errors in 10 games and all have been throwing errors.
"Sometimes when I catch the ball, I don't put my elbow up," Cedeno said Sunday. "That's what I'm trying to do."
Third base coach Chris Speier is working with Cedeno and his throwing motion.
"We've got a routine so I can feel comfortable -- it's like a pitcher," Cedeno said.
"He's conscious of it," Baker said. "The guy's going to be a heck of a player. We're trying to make some moderate changes in his throwing stroke, and that's something that's not easily changed in a short peroid of time when you've been throwing like that most of your life. It just shows you have to keep working on it."
The errors have been aggressive errors.
"Sometimes, I'm so aggressive, I have to learn not to throw everything hard," Cedeno said.
Healthy arm: Scott Williamson has not given up an earned run in his last five appearances after serving one up in his first outing of the year. This is his second season after his second Tommy John elbow surgery, which is when most pitchers finally feel good.
"It's not 100 percent back," Williamson said Sunday of his arm, "but I think I have a little bit more life in the fastball in that last-second jump of the ball before it gets to the plate. I think the miles per hour and stuff will come the more I throw this year. Hopefully, by midseason I think we'll see a jump when it gets warmer.
"I feel a little more confident in my stuff, even though it's somewhat the same as last year," he said. "It's a litle sharper on the breaking balls and a little better accuracy. It's a long process. You take it day to day and some days you feel better than others."
Williamson was the key on Friday, pitching two scoreless innings against the Pirates. In 17 appearances last year, he did not pitch more than one inning, and had not gone two since May 1, 2004. As much as the right-hander likes seeing the starters go deep, Williamson needs regular work.
"I've always pitched better and gotten stronger the more I pitch and the more innings I pitch," he said. "The times I've had problems is when I've sat for four or five days in between or gotten an inning sparingly. If we can stay in the same program, I feel very confident I'll have a real good year.
"I've always been kind of a touch-and-feel kind of guy with the slider and the split. The more I get out there, the better feel I'll have for them."
Aches and pains: John Mabry did not come out of Friday's collision with Sean Casey pain-free. Mabry experienced some spasms in the back of his neck and shoulders and was not available on Saturday. He was available to pinch-hit on Sunday.
Casey, the Pirates first baseman, is sidelined six to eight weeks with a back injury.
Meanwhile, outfielder Jacque Jones has been running well, and is expected to be ready for the Cubs' series in Los Angeles, starting Monday. Jones has been sidelined with a strained right hamstring since Tuesday, and he did pinch-hit in the ninth inning Sunday.
"We're very pleased with what we see," Cubs athletic trainer Mark O'Neal said of Jones.
Minor matters: Rich Hill gave up one run on four hits and one walk over seven innings while striking out 10 to help Triple-A Iowa beat Round Rock, 3-1, on Saturday. Hill recorded at least 10 strikeouts in a game for the fifth time in his Minor League career. Felix Pie was 2-for-4 with two doubles and two RBIs, batting leadoff. Aardsma threw two no-hit innings in relief and Buck Coats was 2-for-4. ... Juan Mateo gave up two runs on four hits and three walks over five innings in Double-A West Tenn's 2-1 loss to Tennessee. Micah Hoffpauir hit a solo homer and Adam Greenberg was 2-for-3. ... Grant Johnson gave up four runs -- two earned -- on eight hits over 5 1/3 innings as Class A Daytona beat Palm Beach, 10-4. Issmael Salas had three hits, Joe Simokaitis and Jake Fox each had two. Fox hit a three-run homer. ... Mitch Atkins threw five shutout innings in Class A Peoria's 2-1 loss to Quad Cities. Davy Gregg was 3-for-4.
Extra bases: The Cubs will get a first-hand look at Kerry Wood, who is scheduled to meet the team in Los Angeles on Monday and throw a side session. Wood is rehabbing from arthroscopic surgery on his right shoulder and threw a three-inning simulated game on Saturday in Mesa, Ariz. Mark Prior (shoulder) threw on the side Sunday. He has yet to get the go-ahead to face hitters. ... Juan Pierre has four stolen bases in 11 games. He had two total in April 2005. ... Matt Murton was 0-for-4 Sunday to end his hitting streak at five games. He's 9-for-23 in his last six games. ... Freddie Bynum Jr. made his first start on Sunday, and batted second for the first time in his young career. His father, Fred Bynum Sr., has some baseball in his genes. Son Freddie says dad was a very good softball player. ... Looking ahead to the Cardinals series next weekend, it appears that the Cubs will miss St. Louis ace Chris Carpenter. He's scheduled to pitch Wednesday against the Pirates. ... Jones was the latest Cub to be seen wearing a blue camouflage shirt under his jersey to match Pierre.
On deck: The Cubs head west to open a three-game series against the Dodgers. Greg Maddux will open the series Monday night against Brett Tomko. Carlos Zambrano will start Tuesday, looking for his first win of the season, and rookie lefty Sean Marshall will go Wednesday against Brad Penny.