Lester was examined after an abbreviated outing on Thursday against the Reds, and Chicago was encouraged that there was no structural damage, but only tightness in the pitcher's side and fatigue in his shoulder.
"His arm is tired, which is understandable if you look at the load he's carried," Cubs president of baseball operations Theo Epstein said. "Pitching seven months the last couple years and taking the ball every fifth day, there comes a time when all pitchers need a breather, and this is his time.
"[Lester] told us he's been dealing with this for a good bit. This is probably inevitable. I'm glad he's getting the break before anything serious happens."
Lester had to leave Thursday's game after throwing 46 pitches over 1 2/3 innings because he felt tightness in his left lat. Cubs manager Joe Maddon also noticed a drop in velocity and that Lester wasn't able to finish his pitches.
"Those couple miles an hour makes all the difference in being an effective pitcher," Maddon said.
Lester was sidelined with a lat strain in 2011, but he said this wasn't as severe, and he expected to be out a minimum amount of time.
"The big thing is the overall performance wasn't there," Lester said. "It's something we've tried to manage for a while and get through. It got to a point where you're doing a disservice to your team by not being able to perform.
"It [stinks] going on the DL, but at the same time, I wasn't helping going out there."
Lester didn't want to predict when he'll return.
"I don't expect it to be long, but that being said, I don't want to put a timetable on it either," he said. "Obviously, the quicker the better, but I don't know. We'll play it by ear."
Mike Montgomery will take over in the rotation for Lester, who was not expected to miss more than two starts.
"We're in a fortunate situation having a guy like Monty who can step in our rotation and do a good job for us," Jake Arrieta said. "Having a guy like him who is versatile and can provide nice results is a boost for us."
Lester needed six pitches to retire the side in the first, but the second inning was a different story. The Reds rattled off four straight hits to open the inning, and Lester also served up a three-run homer to Joey Votto. Adam Duvall reached on a fielding error by third baseman Kris Bryant, and Eugenio Suarez drove him home with a double. Lester then signaled to the dugout, and he was pulled.
The Cubs did rally to tie the game at 9, but the Reds eventually won, 13-10.
Lester is the third Cubs starter to go on the disabled list this season, but he is the first in the second half. Kyle Hendricks and John Lackey both missed time before the All-Star break because of injuries.
"He needs a breather," Epstein said of Lester. "He needs a few days without throwing to regain the strength in his shoulder. He just needs a little bit of a breather."
Fantasy spin | Fred Zinkie (@FredZinkieMLB) Though he has posted a 4.37 ERA through 26 starts of an inconsistent 2017 campaign, Lester was a mixed-league ace a year ago (2.44 ERA, 1.02 WHIP), and he should be retained in all leagues for what should be a short disabled-list stint. Shallow-league Lester owners can check waivers for a replacement such as Patrick Corbin, Luis Castillo or Eduardo Rodriguez. Meanwhile, those in deeper leagues can take a look at Reynaldo Lopez, Brandon Woodruff or Tyler Skaggs.