John Lackey appeared relaxed, almost carefree, as he made his way to the field before the Red Sox opened Fenway Park yesterday. So far, as least, the medical report on his right arm is as positive as can be. An MRI administered Sunday revealed only inflammation, “not any detection of a tear,” in Lackey’s right biceps, according to manager John Farrell. Thus, the Sox have not yet decided whether to put the veteran right-hander on the 15-day disabled list, although the move remains a possibility. Lackey isn’t expected to make his next start, which would be Friday night against the Tampa Bay Rays. But thanks to a day off today, the Sox don’t necessarily need a fifth starter until at least Sunday. If Lackey can’t pitch by then, right-hander Alfredo Aceves remains the frontrunner to take his turn. “We’ve got some time before we settle on our approach,” Farrell said. Making his first start in 18 months after coming back from Tommy John elbow surgery, Lackey grabbed his right arm and walked off the mound in the fifth inning Saturday in Toronto. Farrell characterized Lackey as having “improved,” but he also said the pitcher still is in a “48-to-72-hour recovery period.” Lackey deferred to Farrell for comment on the situation.