"It was just kind of going on all day," said Betts, who was scheduled to be re-evaluated on Thursday morning. "I had a little stiffness, and it gradually got a little worse."
"There's some swelling in that right calf; there's tightness, obviously," Farrell said.
Betts went 2-for-4 with an RBI on Wednesday before exiting his first game out of the three-hole. He doubled twice in a game for the sixth time this season, all at Fenway Park. Betts' 34 doubles rank second in the American League, behind teammate David Ortiz, who also left the game early, hobbling off the field during a ninth-inning at-bat after fouling a ball off his right shin. Ortiz's X-rays were negative, and he is day to day.
Betts is the only player in the Major Leagues with at least 75 RBIs, at least 80 runs and a batting average of .300 or higher.
Dustin Pedroia, who hit leadoff for the first time this season, is eager to get both Betts and Ortiz back on the field. Pedroia went 2-for-5 with a double, extending his home on-base streak to 23 games.
"Obviously, those are two of our best players, so we've got to get them back out there," Pedroia said. "Find a way to get them back out there, feeling healthy and better."
Deesha Thosar is a reporter for MLB.com based in Boston. This story was not subject to the approval of Major League Baseball or its clubs.