(CBS) Cubs left-hander Jon Lester (shoulder fatigue) and catcher Willson Contreras are each doing “extremely well” in their recovery and rehab from their respective injuries, but a return isn’t imminent for either players, president of baseball operations Theo Epstein said on 670 The Score on Thursday morning.

Lester is scheduled to throw a bullpen session Friday. He’s already missed one start and will miss at least one more, with the hope that he could return the first week of September.

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“When a pitcher has a little bit of a fatigued shoulder, you rest him for a few days, and then things feel better,” Epstein said in an interview with Brian Hanley and Joe Ostrowski. “There’s a tendency to say, ‘Oh, great, he just miss one start and hop right back out there.’ Usually if you do that, there’s a price to pay down the road. And every day of no throw or every day of light throwing and allowing him to build up that shoulder pays dividends down the road. He’s in a really good place and getting a lot stronger.

“It seems that we should really him to get the position to not just come back and pitch but come back and be completely himself and be able to be himself with the hope that we pitch through October. The good news is he avoided any kind of real injury here. It’s truly a fatigue situation where he just needs a little bit of a breather to get stronger.”

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Mike Montgomery has replaced Lester in the rotation and picked up the win at Cincinnati on Wednesday night. Lester is 8-7 with a 4.37 ERA and 1.27 WHIP on the season.

Contreras suffered a moderate hamstring strain in the second week of August and is expected to be out until at least mid-September.

“He’s bouncing around, doing a ton of things on the field and doing really well,” Epstein said. “He’s a smart kid as well and understands just you feel like you can come back and play one baseball game doesn’t mean you’re in a position where you can come back and catch in a pennant race every day.

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“We’re just being smart, but both guys are progressing really nicely.”