Just as he does before every game, Ortiz was doing some sprints on the field shortly before Game 1. Only this time he didn't feel right, and he had to come out of the lineup. He also missed Game 2.
Though the club does not expect that Ortiz will have to go on the disabled list, the injury will be treated conservatively.
Ortiz was examined by the team's medical staff during Game 1.
"He got an MRI," said manager Terry Francona. "It showed bursitis in his heel. We're going to get him out of his spikes for a couple of days and get him in a little bit of a walking boot, just to kind of alleviate some of the pressure.
"We'll keep re-evaluating every couple of days. We want to get him off of [the foot] for a little while. It's not going to be DL. We're going to have to keep him out of there for a little bit. We don't know what that is."
Following Wednesday afternoon's contest at Fenway, the Red Sox hit the road for an eight-game trip through Kansas City and Texas.
"It was bothering him on the [last] road trip, and when he went out to run his sprints before the game, it kind of grabbed at him a little bit," Francona said.
The Red Sox managed just three hits in each game of their doubleheader, sans Ortiz, who is hitting .300 with 24 homers and 79 RBIs.
In Game 1, Jed Lowrie served as the designated hitter in Ortiz's place. Carl Crawford got to rest his legs by DHing Game 2, with Darnell McDonald getting the start in left.
The Red Sox were also without Marco Scutaro for the doubleheader, as he's been dealing with back discomfort. Mike Aviles started both games at short.
The Sox are taking a "wait and see" approach with Scutaro.
"[His] back's a little stiff, so that's kind of where we're at," said Francona before Game 1. "It's an early day, so he'll be getting treatment and we'll see how he does later."
Kevin Youkilis has also been dealing with a nagging back and didn't play Game 2. The Red Sox hope he will start on Wednesday against David Price.