The two-time National League All-Star missed all of last season with the Brewers following a pair of microfracture knee surgeries and McClendon said his latest injury further slowed his progress.
"Corey is getting better," McClendon said. "It's a little frustrating for him right now. He's a little jumpy. Coupled with the fact he missed so much time, then he got hurt again when he was starting to swing good, he's just got to slow down and allow the game to come to him a little bit.
"I told him that his last at-bat [on Thursday]," said McClendon. "Just slow down a little and stop trying to force the issue. Nobody cares more than him. Those are tough at-bats. But I do see some progress and I think if he can stay healthy and get out there on a consistent basis, he's going to make a good run for us down the stretch here in the second half."
Greg Johns is a reporter for MLB.com. Follow him on Twitter @GregJohnsMLB as well as his Mariners Musings blog. This story was not subject to the approval of Major League Baseball or its clubs.