A.J. Burnett cannot put much pressure on his right leg, the result of what he called a grade 1 tear in his calf that had him in a walking boot Friday afternoon.

Burnett, who went on the 15-day disabled list Thursday -- retroactive to Sunday -- said he hurt the calf during his workout routine.

"I was just doing my normal running," Burnett said. "Freak thing. I kind of wish I would have done it goofing off. I was just doing my normal stuff. I went to slow down, decelerate. I felt the pop, tug, pull, whatever you call it."

Burnett said the trainers had not given him a prognosis yet.

"Hopefully I can just get back sooner or later and not turn it into a six-week thing, take my time and keep it a two-week thing," he said.

A grade 1 strain is the least severe form of the injury, but Burnett said it was troublesome.

"I'm getting treatment on it all day," he said. "All day [Thursday], all day [Friday]. I just got to stay off of it. Any time I bend the ankle either way, it aggravates it."

Burnett, the Pirates' opening day starter, ranked third in the National League in strikeouts and eighth in innings pitched before Friday. He had a 3.12 ERA in 891/3 innings pitched, and in his most recent start June 8 he went 81/3 innings.

"It had nothing to do with his last outing," manager Clint Hurdle said. "It's just one of those freak accidents or freak incidents that take place."

"I got a history of getting hurt not doing anything on the field," Burnett joked.