The White Sox placed right-hander Bartolo Colon on the 15-day disabled list with inflammation in his left knee and purchased the contract of left-hander Aaron Poreda from Double-A Birmingham on Tuesday.
Colon is 3-6 with a 4.23 ERA in 11 starts this season. He tied a career-high by allowing four home runs in a loss to Cleveland on Sunday, and has allowed 12 homers in 11 starts this season.
"Colon has a knee problem. He feels it every time he's going to pitch," said White Sox manager Ozzie Guillen, who added that he wants to pace the veteran through the season. "Don't put him in the situation where he has to come here every day and work on it or try to deal with it. Make sure that when he comes back he comes back strong."
Despite making only 36 appearances in the last three seasons, mainly because of injuries, Colon agreed to a $1 million, one-year contract with Chicago in the offseason.
The 22-year-old Poreda was the White Sox's first-round draft pick in 2007. Although he will work from the bullpen, Poreda was 5-4 with a 2.38 ERA in 11 starts with Birmingham.
"I don't think it's going to be a big transition," he said. "I always came out of the bullpen a little bit in high school, college, and when I went to the fall league I came out of the bullpen, so I know how to get ready.
"I know it's different," Poreda said. "You don't have as much time like a starter, but I'm excited to have the opportunity and looking forward to coming out of the bullpen."
Guillen, who's been under mounting pressure, said he won't be patient if Poreda struggles.
"It's about winning. I talked to him and made it real clear. If you don't pitch well you're going to have O'Hare airport next to you pretty soon," Guillen said.
"I asked him if he arrived at Midway or O'Hare and he said O'Hare. I said on the way back Midway is closer. This is to win."