Lester left Thursday's 13-10 defeat to the Cincinnati Reds in the second inning. He allowed nine runs (seven earned) in 1.2 innings.
Lester has struggled this year, going 8-7 with a 3.99 ERA in 25 starts prior to Thursday's game. According to Baseball Reference, his 3.69 FIP is also his highest since 2012.
Prior to the 2017 season, the left-hander showed the durability and consistency that likely gave the Cubs confidence to sign him to a six-year, $155 million deal. For the ninth straight year, he made 30-plus starts, and he logged 200-plus innings for the eighth time in nine years.
Losing Lester will be a big blow for the Cubs, especially if he's out for a prolonged period of time. Jake Arrieta is having his worst season since breaking out with the Cubs in 2014, while Kyle Hendricks has taken a step backward after his third-place finish in last year's Cy Young voting.
Chicago's National League Central lead slipped to only a game with Thursday's loss. Without Lester, the Cubs will have a fight on their hands to claim the division crown for the second year in a row.