After only their first formal workout, the Red Sox have an injured pitcher.

Clay Buchholz sustained what both he and manager John Farrell termed a “mild” strain of his right hamstring while covering first base in pitchers’ fielding drills yesterday. The right-hander doesn’t expect to miss more than five days of workouts, although Farrell hesitated to put a timeframe on his return.

“It’s the best-case scenario,” Buchholz said. “If you had to have a hamstring injury, it’s the best one. I’ll get some treatment on it for the next couple days, and I shouldn’t be out for very long.”

Buchholz missed nearly a month in 2010 when he strained his left hamstring, but he said this injury is far less serious. For starters, it’s concentrated to what Farrell called “the belly of the hamstring” rather stretching down toward the knee.

“It’s completely different,” Buchholz said. “It’s pinpointed and there’s no spread of any kind of soreness anywhere. They said they wouldn’t even classify it as a pulled muscle.”

Nevertheless, it’s hardly ideal for the Red Sox, who are counting on Buchholz to join lefty Jon Lester in anchoring the top of their rotation.

Buchholz has dealt with non-arm-related injuries over the past three seasons. In 2010, he strained his hamstring; a stress fracture in his back sidelined him for the final three months of the 2011 season; last year, he missed a few weeks after being hospitalized for inflammation of the lining of his esophagus.

“I used to be pretty athletic and able to do agile stuff. The last couple years, I’ve proved myself wrong,” Buchholz said with more than a touch of self-deprecation. “I’d rather it happen now than happen when we’re getting ready to throw in games and stuff.”