The team announced the injury after Strop underwent an MRI on Friday.
Strop has been Chicago's closer since Brandon Morrow was placed on the disabled list on July 18. He has 11 saves in 13 opportunities since then and 13 saves on the season. He's 6-1 with a 2.26 ERA over 59 2/3 innings in 60 appearances.
Unless the Cubs can make a trade, manager Joe Maddon said before Friday's game against the Cincinnati Reds that he envisions using a closer-by-committee approach, calling on veterans including Jesse Chavez and Steve Cishek to finish games.
Strop hurt his hamstring while running to first base on a double-play grounder in the 10th inning of the Cubs' 4-3 win at Washington on Thursday. It was just the second at-bat of the season for the 33-year-old right-hander.
Strop pitched 1 2/3 scoreless innings against the Nationals to earn the win. He was set to pitch the bottom the 10th after the Cubs took the lead and was allowed to bat with the bases loaded and one out.
Morrow, sidelined with right biceps inflammation, expects to throw a simulated game on Saturday.
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