Parra jammed his thumb on the ball trying to bunt with Damian Miller on first base. Parra fouled off the attempt but knew right away that something was wrong.
"I looked down and saw some blood through the [batting] glove," Parra said. "It was on the first attempt, and then the second attempt, my mind wasn't right for that one."
Parra popped out to first baseman Derrek Lee on the next pitch and returned to the dugout to show manager Ned Yost the injury.
"He smashed his thumb pretty good, his nail was loose," Yost said. "He didn't have any reaction [after getting hit], he didn't show it. Then, when he came back in the dugout, he took his glove off and showed me, and it was smashed pretty good."
Parra was replaced by Chris Capuano in the bottom of the inning, and the Brewers went on to lose, 5-4, and fall 2 1/2 games behind Chicago in the National League Central.
"It just looks like the nail got lifted up a little bit, so we got some swelling underneath, and blood underneath," Parra said. "So right now, [we've] just got to see how it responds."
The Brewers scored two quick runs in the first, and Parra had a 1-2-3 first, beginning with a strikeout of Alfonso Soriano. But he got in trouble in the second.
Aramis Ramirez walked to lead off the inning and advanced to third on Craig Monroe's double. Mark DeRosa hit a two-run single to tie the game, and the Cubs took a 3-2 lead on Jacque Jones' RBI single.
"I'm extremely disappointed," Parra said. "I'm disappointed not only about [the injury], but the fact that we put up two quick runs, and I wasn't out there throwing strikes the way I normally do. I think I'm more disappointed about that.
"I don't think [the injury] is going to be a big deal," he said. "I would have liked to put in six, seven quality innings, and given us a better chance to win."
Threes were wild for Parra, who gave up three runs, all earned, on three hits over three innings. He also walked three and struck out three. It was the first time a pitcher finished with that line since Sonny Siebert of the Oakland Athletics did it Aug. 5, 1975, according to STATS, Inc.
"Manny's got very good stuff, but he commands it," Yost said. "And he's so young, his stuff is only getting better -- [he has the] chance to be a special guy for us in the future."
Parra entered Thursday's game with a 3.09 ERA, with opponents scoring 10 runs, eight earned, in 23 1/3 innings. He had 23 strikeouts while giving up 22 hits.
"Every time we've thrown him out there, he's pitched very well for us this year," Yost said. "He hasn't had any major eruptions, he's always gone in and done a nice job -- stayed on the attack and thrown strikes. If he can throw strikes like he's done up to this point, it's worth a gamble."