Beckett, who was standing in left-center field, went to one knee after he was struck in the left side of the head. Teammates immediately called for the training staff, and he was escorted off the field with his hand on his head.
The Red Sox said the pitcher did not need to be taken to the hospital and was treated at the park. The team initially said Beckett was OK, but he started to experience concussion symptoms and was sent home to rest. Red Sox manager Terry Francona admitted Beckett was actually in the dugout for the first two innings of Monday's game against the Twins.
"He's exhibiting symptoms of a mild concussion. He's got a headache," Francona said after his team's 7-6 win. "He's been evaluated already and we'll get somebody to look at him tonight, and then when he comes in [Tuesday] we'll check him to see what level of a workout he can do, or should do. We'll go from there."
Beckett is scheduled to start on Thursday, but Francona said he would wait until Tuesday to make a decision whether the right-hander will pitch.
"Let's just wait 'til tomorrow," he said, "and see how he does."
The ball that struck Beckett was hit by Red Sox staffer Ino Guerrero, who was standing in the outfield during batting practice hitting shagged balls with a fungo bat back to a bucket behind second base when one got away from him.
"I think it hit him in the temple," Francona said. "I bet you it felt like a bolt of lightning. That's not really what you're expecting. It's just a fluke thing. Fortunately, it hit Beckett in the head. It's better than hitting him in the shoulder."
When asked what he said to Guerrero, Francona said: "I called him a dumb [expletive]."
Beckett is coming off a disappointing, injury-riddled 6-6 season, in which he had a 5.78 ERA. Beckett, a 1999 first-round pick of the Florida Marlins, is 112-74 for his career, with four shutouts and nine complete games. This is his sixth season with the Red Sox.
Beckett started Sunday night against the Twins and pitched two innings.
Joe McDonald covers the Red Sox for ESPNBoston.com. Information from The Associated Press was used in this report.