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Jon Lester could tell something wasn’t right.

Long known for taking the ball whenever it was his turn to pitch, no matter what — never making fewer than 31 starts in any of the past 12 full Major League Baseball seasons — Lester seemed, to use his phrase, “a little more sluggish.”

During games with the Chicago Cubs. Even between games. And so he wondered: Was he putting in enough work? Is this just what getting old feels like?

“There would be times where I would run out in the fifth, sixth, seventh inning and feel like I hit a brick wall. There were times last year where I would come out of the bullpen and be like: ‘God, did I pitch the game already?'” the 37-year-old Lester, who is now with the Washington Nationals, said Wednesday during a video conference from spring training in West Palm Beach, Florida. “Just thinking that maybe I needed to do a little extra in the weight room. Maybe I needed to run a bit more. Maybe I needed to do 20 extra minutes on cardio or whatever. When in actuality, this thing was slowing me down.”

“This thing” turned out to be hyperparathyroidism, which can affect the amount of calcium levels in the bloodstream and lead to someone tiring easily. So last week, Lester left camp to have what he called very minor surgery to remove one of his parathyroid glands — a scar now runs horizontally at the base of the front of his neck — and he says he already senses a difference in his energy levels.

“The big thing is kind of your brain telling you: You can do something or you want to do something. And I think that’s been a big switch in my head,” said Lester, who signed with Washington as a free agent after spending six years in Chicago. “I feel like since I’ve been back, it’s just the desire to want to work out.”

Lester has been playing catch on flat ground and soon should be able to throw off a mound to work his way into form.

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“He’s done all his leg routine. He’s doing his shoulder exercises. It’s just a matter of getting his flexibility back a bit on his neck and getting some strength back,” manager Dave Martinez said. “But we don’t want to put a timeline on him.”

Lester acknowledged he’s “a little behind the 8-ball” in terms of being prepared for the April 1 start of the season — but he also said it’s helpful to know that he is not the ace of this staff.

He is, after all, part of a rotation that features Max Scherzer, Stephen Strasburg and Patrick Corbin.

“It’s like, ‘OK, if I miss one (start), they’ll be able to pick me up,'” Lester said. “But that being said, I do pride myself on making all my starts, so that’s what we’re going to strive for.”