The Cubs are bracing for the possibility that they could be without their rotation ace for most or all of what’s left of the regular season after Jon Lester left his start Thursday in the second inning because of tightness in his left lat muscle.

Lester, the Cubs’ two-time Opening Day starter and a Cy Young Award finalist last year, was scheduled to see the team physician after the game, including a possible MRI, to determine the severity of damage to the large muscle that extends up from the back into the shoulder area.

He’s expected to go on the disabled list Friday as the Cubs add a pitcher from Class AAA Iowa to bolster a bullpen forced to cover 7⅓ innings of a 13-10 loss to the last-place Reds.

“When a pitcher of his stature is potentially injured, of course you’re a little bit concerned,” manager Joe Maddon said. “But I’m not going to jump to any kind of negative conclusions.”

The Cubs are expected to shut Lester down for at least a short DL stint, given how critical their postseason horse — and three-time World Series champ — is to their playoff plans.

The nature of lat injuries and the recent history with pitchers who have suffered various degrees of them suggest a four-to-six-week range for a strain — potentially three months or more for a tear.

Pirates pitcher Gerrit Cole, for instance, missed almost seven weeks in 2014 because of a lat strain.

Athletics right-hander Sonny Gray suffered a strain in spring training, opened the season on the disabled list and didn’t make his first start until a month into the season.

Mets left-hander Steven Matz missed two months in 2015 for a Grade 2 tear and Mets teammate Noah Syndergaard hasn’t pitched since April 30 after suffering a similar tear.

The Cubs’ division lead was down to one game over the Brewers after the loss, with six weeks and 42 games to play.

“He’s our ace. He’s our guy,” said left-hander Mike Montgomery, who pitched 4⅓ innings of emergency relief (61 pitches) and is expected to take Lester’s rotation spot indefinitely.

“Hopefully, he’s going to be all right,” said Montgomery (2-3 with a 5.13 ERA in eight starts this season). “If they need me to go in five days I should be ready.”

Jon Lester asks for trainer, leaves game, talks to Bosio in the dugout. Hurt? Pulled himself out? #Cubs pic.twitter.com/K7kQJpFoEo — Aldo Soto (@AldoSoto21) August 17, 2017

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The Cubs are optimistic Lester’s injury isn’t serious. The stoic left-hander, who isn’t quick to acknowledge aches or nagging ailments, wasn’t in visible pain, even during an 11-batter, nine-run second inning.

He motioned to the dugout after his final batter and quickly left the field with assistant athletic trainer Ed Halbur.

“Obviously something wasn’t right, because you saw the velocity [was down],” Maddon said. “The cutters were like 84 [mph], 85. That’s not quite right. It might have been bothering him from the beginning. He didn’t say anything though.”

Lester pitched a 1-2-3 first inning before giving up four consecutive singles to open the second, then a two-out walk, followed by another single and a home run.

“The velocity was down on everything a little bit,” Maddon said. “The pitches were getting in, but they weren’t finishing. They were able to fight them off enough to get over the infielders’ heads. He was lacking that last thing that you normally have on your pitches that permit you to not have that happen or get even weaker contact.”

The Cubs’ rotation has been the backbone during the second-half push. The starters are 16-5 with a 2.97 ERA since the All-Star break until Thursday. The rotation had a 4.66 ERA before the break.

Lester has been a leader for the rotation since signing a six-year $155 million deal before the 2015 season.

During last year’s postseason run to the World Series championship, Lester was co-MVP of the NLCS and was the Cubs’ Game 1 starter in each series.

Follow me on Twitter @GDubCub.

Email: gwittenmyer@suntimes.com

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