Farnsworth said Wednesday that an MRI revealed a strain of a muscle that's causing soreness in his elbow. The reliever, who had 25 saves in 31 opportunities in 2011, said it's the same problem that shut him down for a portion of last September.
"I just have to give it some time to calm down," said Farnsworth, who went 5-1 with a 2.18 ERA over 63 appearances last season.
"It's weird. Sitting around it doesn't really hurt. Playing catch doesn't hurt," Farnsworth said, adding he occasionally feels discomfort when he's finishing pitches.
"Sometimes I'd feel it, sometimes I wouldn't," the right-hander said. "It was more aggravating than anything trying to figure out what it is. ... Hopefully we got it figured out this time."
Manager Joe Maddon would not speculate on how long Farnsworth might be sidelined. The pitcher was on the 25-man roster submitted Wednesday, but that could change by Opening Day.
The Rays will open the season without outfielders B.J. Upton (lower-back soreness) and Sam Fuld (right wrist surgery) and reserve catcher Robinson Chirinos (concussion), who were all placed on the 15-day disabled list Wednesday.
Information from The Associated Press contributed to this report.