Grabow entered Monday's game against the Pittsburgh Pirates in the seventh inning and retired the first batter, then gave up an infield single to Ronny Cedeno. Paul Maholm squared to bunt on a 1-1 pitch, and Grabow finished the pitch awkwardly. Athletic trainer Mark O'Neal went to the mound and Grabow immediately headed to the clubhouse.
"It's pretty bad," Grabow said. "It's something I thought I could pitch through and it got worse. I'll go get it checked out [Tuesday]."
Grabow has received a cortisone shot already this season in the knee and was on the disabled list from May 31 to June 15 because of it. In his previous outing Thursday, he gave up one hit over two innings against the Mariners.
"I tried to pitch through it and today it felt a little different, and it kept getting worse and worse to the point where I couldn't take it any more," Grabow said.
The Cubs' bullpen has been busy after Carlos Zambrano's abbreviated outing on Friday. Right-hander Jeff Stevens will be added on Tuesday as Zambrano is moved to the restrictted list. The Cubs will add another pitcher to fill Grabow's spot on the 25-man roster as well.
If the Cubs opt for a left-hander, it could be James Russell, who began the season with the big league team. Russell has given up seven runs on 11 hits and four walks over 11 innings in five games with Triple-A Iowa, striking out 10. Left-hander John Gaub has a 4.33 ERA in 27 games with the Iowa Cubs, serving up 13 earned runs on 24 hits and 16 walks over 27 innings while striking out 37.
If the Cubs opt for another right-hander, Jeff Samardzija is an option. He has a 1.88 ERA in 20 games, including three starts at Iowa. He's given up eight earned runs on 24 hits and 22 walks while striking out 33 in 38 1/3 innings. He has started in three of his past four outings but that change in assignment was because of injuries or roster moves. Right-handers were batting .173 against Samardzija while lefties were hitting .189.
Grabow was obviously upset at the injury.
"It's tough," he said. "I felt like I was starting to get better. My knee hadn't been acting up too much and started to get on a roll."