He aggravated a previous injury on a check swing in the fourth inning of a 6-3 victory Thursday night over the Minnesota Twins. Billy Butler hit for Hosmer in the sixth and X-ray's revealed the fracture.
Hosmer was hit by a pitch by Jon Lester on July 20 at Boston and did not start six of the next seven games.
''Very frustrating, bad timing,'' Hosmer said.
Hosmer, who will go on the disabled list, had a 16-game hitting streak before Lester's pitch hit him.