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Mike Leake has made one start for the Giants since San Francisco acquired him on July 30.
It's unclear when he will take the mound again.
Leake will not pitch against the Cubs on Friday because of a mild left hamstring strain, the Giants announced Thursday afternoon.
Ryan Vogeslong, who last started on July 11 against the Phillies, will get the ball in his place.
Leake allowed two runs over 6.1 innings last Sunday against the Rangers.
According to reports, he felt tightness while doing his normal conditioning in Atlanta.
Mike Leake scratched for tomorrow. MRI showed mild left hamstring strain. Vogelsong will start.— Andrew Baggarly (@extrabaggs) August 6, 2015
Leake felt tightness in normal conditioning in ATL. Thought it was a cramp but didn't subside. No DL move, as SF hopes it's just one start.— Andrew Baggarly (@extrabaggs) August 6, 2015
Bochy hopes Leake misses just one start. Leake thought it was cramp at first. Felt it during conditioning in Atlanta.— John Shea (@JohnSheaHey) August 6, 2015
Giants alter pitching rotation: Vogelsong to start Friday for Mike Leake, who felt twinge in L hamstring while doing regular conditioning— Chris Haft/SF Giants (@sfgiantsbeat) August 6, 2015