Travis Hafner had been able to shake off his recent history of injuries since joining the Yankees, but it was easy to become alarmed when he was forced to miss last night’s 4-3 win over the Mariners and get an MRI exam on his ailing right shoulder. The Yankees’ concerns were allayed when the test showed only tendinitis, meaning Hafner could be back in the lineup as soon as tonight after getting a cortisone shot. Hafner pointed to getting hit by a pitch in the area on April 29 as the original cause of the injury and was pleased to learn it wasn’t more serious. “Hopefully this is nothing serious,” Hafner said. “I’ll come in [today] and see how it feels.” In 32 games, Hafner has six homers and 18 RBIs, as well as an OPS of .893 — a number he hasn’t reached for a full season since 2006. Of course, he has played more than 100 games just once in the last six seasons, constantly done in by injuries.