In an inning of work, he allowed two runs on a hit and two walks, while striking out two.
This is actually the second time the Chicago Cubs ace has allowed nine or more runs in an inning.
Joey Votto had a three-run homer in the outburst.
In 2017, Lester has an 8-7 record with 4.37 ERA with 155 strikeouts and a 1.27 WHIP.
Trump says Kim made 'wise and reasoned' decision
The president added, "They will be met with fire, fury and frankly power the likes of which this world has never seen before".
Lester gave up 10 runs in two-thirds of an inning at a start versus Pittsburgh on July 9. The Cubs' rotation has been an issue all season, and an extended absence from Lester won't help. Prior to Thursday's outing, Lester had strung together six strong starts, averaging over 6 1/3 innings per with a 3.26 ERA.
For now, the injury is not considered serious, but the Cubs are being cautious and trying to make sure Lester is healthy for the playoffs, should they get there. Lester was taken for examinations at Northwestern Memorial Hospital during the game. But with the NL Central as tight as it is, the Cubs can't afford Lester to miss any significant time.
The Cubs go into Friday's game against the Toronto Blue Jays with a one-game lead over Milwaukee and 1 1/2 games over St. Louis.