Lohse, who hadn't pitched since May 23 and was pushed back in the rotation twice, reinjured the forearm while diving to field -- and then throwing to first base -- on an infield single by Reds pitcher Johnny Cueto.
He first injured the arm when he was hit by a pitch while batting against the Royals on May 23. He twice attempted to throw bullpen sessions that he had to cut short due to discomfort in his forearm. However, after a successful bullpen session on Sunday in San Fransisco, he declared himself healthy and ready to go.
After taking 11 days to heal the first time, it seems that a trip to the disabled list could possibly be in Lohse's future.
"We'll see what's going on in there," Lohse said. "It's going to have to show some big improvement here in the next couple of days, I'm sure, [to stay off the DL].
"If we have to do that to let it rest, then we do. But I'm going to do everything I can to get out there when I can."
Lohse said that he felt fine up until that point in the third inning, but catcher Yadier Molina noticed problems with his starter much earlier in the game.
"I noticed since the first inning, but I didn't want to say anything," Molina said. "He wasn't very close and wasn't keeping the ball down. I was wondering if something was bothering him.
"When you see a guy who can control and throw strikes, throw like that with the fastball up, way up, there was something wrong with him."
Lohse gave up three hits in the first two innings including a long home run by the Reds' Laynce Nix. Manager Tony La Russa and head athletic trainer Barry Weinberg first came out to check on Lohse in the second inning, but he remained in the game at that point.
"It looked like he was getting the ball up and was having trouble getting it down," La Russa said. "I asked him, but he said he was just rusty. But when he dove on the bunt play, he felt like he reaggravated it. So we will have to just wait and see.
"When he threw the ball to the plate, you could tell it wasn't the same."
Lohse, who has battled a series of nagging injuries throughout the season, stayed in the game after Cueto's bunt and threw four pitches to the next hitter, Jerry Hairston. After the fourth pitch, La Russa and Weinberg came out from the dugout again and Lohse was removed soon after.
"It's obvious when you throw a 78 mile-per-hour fastball that something's not working in there," Lohse said. "I couldn't get the ball anywhere near where I wanted to. [It was a] cramping, burning sensation. It's hard to force yourself to release it and do the proper things that you need to do to get the location and everything else.
"I've never had this many problems. It would be one thing if it were an arm problem from actually throwing, but to get hit, and have this stuff happen, that's what's really irritating."
B.J. Rains is an associate reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.