Beckett flew back to Boston on Tuesday and was examined by the team's foot specialist, Dr. George Theodore of Massachusetts General Hospital.
Red Sox medical director Dr. Tom Gill issued a statement that the club released during the first inning of Tuesday night's 14-0 win over the Blue Jays.
"Josh was evaluated today at the Massachusetts General Hospital by the Red Sox medical staff, including team foot and ankle specialist, Dr. George Theodore," Gill said. "Josh experienced pain in his ankle while pitching last night. His examination was consistent with an ankle sprain. An MRI was performed that confirmed no other injury to his ankle tendons, or his Achilles tendon. We will re-evaluate his symptoms and availability later this week."
Beckett will miss his scheduled start on Sunday against the Rays.
Beyond that, the Red Sox will just wait and see how the righty responds.
"I don't know that we really need to speculate," said manager Terry Francona. "We just need to get him [healthy]. When things like that happen, let's let this thing settle down. Let's let the medical staff work and we'll get him back whenever he's supposed to be."
Missing a start or two would be a small price to pay for Beckett and the Red Sox, considering what the All-Star righty means to the team down the stretch. If there had been ligament damage, Beckett could have been sidelined for weeks.
The medical reports came back about as positive as they could have.
"I'd say in general, yeah," Francona said. "You saw the statement. It's consistent with a sprain and there's nothing else going on, so I would say, yeah."
This weekend against the Rays, the Sox will pitch John Lackey on Friday, followed by September callup Kyle Weiland on Saturday and then Jon Lester in the road trip finale Sunday afternoon.
Weiland made starts for the Red Sox on July 10 and 19 -- both against the Orioles -- going 0-1 with an 8.10 ERA.
"It's a great opportunity and it's nice they chose me to take on that game," Weiland said. "I've just got to stay within myself and look back and learn from the last couple of starts and understand that the same pitches that get guys out in Triple-A get guys out here. Try not to do too much and stay within myself. Hopefully I got some things out of the way early on. It's just a learning process. I'm looking forward to it."
Beckett suffered the injury in the fourth inning of Monday's game after pitching 3 2/3 scoreless innings. A fall in the bullpen during warmups -- when Beckett was simply trying to change his shirt -- may or may not have led to the ankle sprain.
For the season, Beckett is 12-5 with a 2.49 ERA in 27 starts.