CINCINNATI -- The Chicago Cubs watched their best pitcher in June walk off the mound after throwing a warm-up pitch before the bottom of the second inning on Friday, as veteran Cole Hamels left the game with an injury to his "left side," according to the team.

Hamels, 35, was still under evaluation.

The left-hander gave up a first-inning run on a walk and two hits to the Cincinnati Reds but couldn't start the second, as he indicated to manager Joe Maddon he was done, and then continued into the Cubs' dugout.

Hamels is a candidate to make the All-Star team, as well as for pitcher of the month, as his June ERA was 1.00 going into the game. Overall, he was 6-2 with a 2.92 ERA.

Hamels had been pitching through some back issues in recent weeks, a source told ESPN, but it's unclear if that's related to him leaving Friday's game.

Lefty Mike Montgomery took over for Hamels and was promptly greeted with a first-pitch home run by Phillip Ervin.