According to Jason Beck:
The Tigers' regular left fielder did not progress as much as he hoped with his sprained ankle. He was able to take batting practice but not take fly balls. At this point, a return next week could be more likely than this weekend.
Monroe has been semi-struggling this season, batting .236/.276/.418 with 10 homeruns and 28 RBI. He hurt his ankle in the White Sox series earlier this week.