CHICAGO -- Jose Contreras remains on target to make his first second-half start on Tuesday night against the Rangers.

But White Sox manager Ozzie Guillen qualified the assignment with the fact that Contreras still has stiffness in his right arm, which is where he was hit by a Ronny Cedeno line drive during a victory over the Cubs on June 27. According to White Sox athletic trainer Herm Schneider, the stiffness has affected Contreras' pitch selection.

Guillen added before the contest that the team didn't find out about the stiffness until Contreras' last start against Kansas City.

"Right now, he should start," Guillen said. "But I don't want anybody out there to perform just to perform.

"If you're not 100 percent, if you're not sure you can go, you're not going to go. If you tell me, 'Oh, I'm 90 percent ... ,' then we'll shut you down and put somebody else in there. I don't want to send people out there just to ...

"It's about the ballclub. It's about the team," Guillen added. "It's about winning. If you're not ready to help us, we're not going to use you."

Contreras threw on the side on Saturday, but Guillen said any further decision wouldn't be made until the team checked on his status on Sunday. Contreras has allowed 36 runs on 63 hits, including nine home runs, over his last 37 2/3 innings covering seven starts.

In order to get Contreras back toward the form he showed at the beginning of the season, when he had a 2.76 ERA as of June 5, Guillen could skip the right-hander on Tuesday. Guillen's goal is to have his staff at its best come August and September.

"Right now [being in first place], we have the luxury," Guillen said. "Whoever pitches better in August and September, that's the team that has the best chance."