Hitting for himself with one out and the bases loaded in the top of the 10th inning with the Cubs holding a 4-3 lead over the Washington Nationals, Strop hit a sharp grounder to third baseman Anthony Rendon.
Rendon threw home to catcher Matt Wieters for the second out, then Wieters threw to first in a bang-bang play that was initially ruled an out, and then confirmed after the Cubs challenged.
After Strop lunged for first base on his final step, he injured his hamstring, hobbling his way into shallow right field. As the play was being reviewed, he limped slowly back toward first base, visibly frustrated as he was met by manager Joe Maddon and team medical staff.
After the game, which Chicago held on to win 4-3, Strop said he expects to miss an extended period of time.
"Right now it's sore," said Strop, "and it's obviously going to take two weeks, at least, for me to get back on the mound."
The 33-year-old hurler, who's slated for an MRI on Friday, has been instrumental for a Cubs team that's been without closer Brandon Morrow (biceps) for almost two months and is trying to stave off the Milwaukee Brewers in a tight National League Central pennant race. Since Morrow hit the disabled list on July 19, Strop has a 1.77 ERA and 0.84 WHIP, with 11 saves in 13 opportunities.
Strop entered Thursday's game with one out in the bottom of the eighth inning of a 3-3 tie. He got out of that inning, then pitched a scoreless ninth to force extra innings.
Javier Baez drove in the go-ahead run in the top of the 10th on a bunt single. Following a walk that loaded the bases, Maddon decided to let Strop hit for himself, hoping that the righty could close out a game in which the Cubs had already used eight pitchers.
"If we scored, he was going back out," said Maddon of the decision to leave Strop in the game instead of lifting him for a pinch-hitter. "If you don't score, he wasn't. That was it. We scored. Listen, he hit the ball hard. This guy can swing the bat a little bit. That wasn't a fluke. It's just the fact he tried to beat it out, almost did and you can never fault an athlete for competing."
Strop said that if Wieters hadn't been so deliberate with his relay throw to first base, he might not have pushed so hard to beat it out.
"I was on a good pace, but not like all-out," said Strop, who was 0-for-3 lifetime as a hitter entering Thursday's game. "Then when I saw they hadn't thrown the ball, that last step, trying to reach the bag, that's the one that got me."
With the win, the Cubs extended their lead over the Brewers to 1 1/2 games. They return home to host Cincinnati for three games, then fly to Arizona for three more before an off-day next Thursday. By that time, they will have endured a grueling stretch of 30 straight days in which they had a scheduled game.
Their win over Washington came on a day that was supposed to be free, but ended up being a makeup of a rained-out contest from this past Sunday. Between the taxing schedule and the injuries to his relievers, Maddon was unsure who would serve as closer when his team faces the Reds on Friday.
"I have no idea," said the Cubs' skipper. "All these guys are on fumes."