Lohse was on the St. Louis team flight to San Diego on Monday, but told The Associated Press he didn't expect to make his next scheduled start, which would be Friday in Chicago against the Cubs.
"I don't think so," Lohse said. "I really don't know what tomorrow is going to bring."
Lohse is scheduled for further medical exams to determine the extent of the injury. The team said there's no timetable for the right-hander's return.
Lohse had two MRIs on Monday, one before exercising and a second after working out.
"It was enough to show the inflammation," Lohse said. "I basically got the feeling I would get from throwing. Something is going on."
Lohse is 1-4 with a 5.89 ERA. He allowed six runs in 3 1/3 innings on Saturday against the Angels, and said after the game that he often experienced a cramping sensation in the forearm. Lohse was 4-1 last season before being hit by a pitch on the forearm on May 23, and won only two games the remainder of the season.
In 2008, Lohse was a 15-game winner in his first season with the Cardinals.
Lohse is likely to join Brad Penny (upper back) on the 15-day disabled list. Like Penny, Lohse had not mentioned that his 2009 symptoms had returned prior to Saturday's start, but problems in his latest outing convinced him he had to tell the team.
"I learned my lesson last year about trying to pitch through things," he said. "I need to get it figured out."
The Cardinals promoted rookie right-hander P.J. Walters from Triple-A Memphis on Saturday when Penny went on the DL, and Walters worked four innings of relief that day. Among the candidates to replace Lohse are Adam Ottavino, Lance Lynn and Evan MacLane, none of whom have major league experience, and perhaps left-hander Rich Hill.
St. Louis had one rookie in the rotation, Jaime Garcia, before the Penny injury.
"If we had Kyle and Brad down deep you wouldn't have this feeling in your gut that 'Wow, it's going to be a tough time," manager Tony La Russa said. "We've just got to tough it out.
"Somebody will come up and do a good job for us, one of these young pitchers will do well."