A stress reaction is a contusion, and it was confirmed by an MRI, done Wednesday. Parker will require treatment as if he had a stress fracture and won't be allowed to pick up a baseball for 30 days. He last pitched Friday against the Giants, has appeared in four games, and has not given up a run over 4 1/3 innings.
"He said it had been bothering him since Houston [May 21-23], and he just kept trying to throw through it. It got a little bit worse all the time, and he finally mentioned something yesterday," Cubs manager Dale Sveum said.
Corpas made his Cubs debut in Wednesday's 8-0 loss to the Brewers, striking out two in one scoreless inning. It was his first Major League action since 2010.
Corpas appeared in 19 games at Iowa, and in May compiled a 2.33 ERA in 10 games, giving up 11 hits and walking four over 19 1/3 innings. He struck out 13 in that stretch, and opponents hit .167 against him. Right-handers batted .161 against him this season.
Sveum said Corpas will be used in matchups against hitters who struggle against pitchers who throw sinkers.
"It's another mix to the puzzle that we have who's a little bit different than who we have out there," Sveum said.