CLEARWATER, Fla. — Jake Arrieta said he knew something was wrong with his left knee last summer, when pain flared as he left the mound to cover first base on a grounder or left the batter’s box and ran down the line.

He suffered the injury sometime in May, but Arrieta, whom manager Gabe Kapler labeled “a tough individual,” kept pitching and kept his pain hidden.

“It wasn’t enough to keep me off the mound,” Arrieta said Thursday at the Carpenter Complex.

The injury, Arrieta learned last month, was a torn meniscus. He said he heard “a couple pops” during an offseason workout, had an MRI, and flew to Philadelphia for surgery. He was walking that day and resumed training five days later. The Phillies did not know about the injury until Arrieta needed the surgery, Kapler said.

“The timing actually was really nice that it happened when it did,” Arrieta said. “If it would have happened now or two weeks ago, I might have had a setback of some sort. But since that procedure is pretty minimal — instead of repairing the meniscus, they just clean out the tear — since that’s the case, we’re in a good position.”

Arrieta said he has thrown three or four bullpen sessions in camp and will throw again Friday. Arrieta, Kapler, and general manager Matt Klentak each said that the right-hander will be ready for the start of the season.

“You know we stress as much communication as possible,” Kapler said, when asked whether he wished Arrieta would have told the team last season about his injury. “Players who can play through some discomfort, those are things that we respect. Especially as a coaching staff, we always stress that the more communication, the better. We can make better decisions when we have all the information.

"But it’s important to note that we understand that it doesn’t always work out that way. We’re glad that Jake got it cleaned up, and he’s already off the mound a couple of times, and he’s delivering the ball well and moving around great. We have no concerns about his ability.”

The injury, Arrieta said, is an explanation for his struggles in the second half. Arrieta’s 5.04 second-half ERA was nearly two runs higher than his first-half mark. His ground-ball rate decreased in the second half, after he used his sinker so effectively in the season’s first two months to induce groundouts.

The sinker’s velocity and movement decreased after Arrieta said he felt his injury, according to PitchF/X data, perhaps providing the best evidence for the effect a torn meniscus can have on a pitcher. Opponents registered a .311 slugging percentage against him through May and .452 for the rest of the season.

“I knew something was going on in there. I wasn’t sure what,” Arrieta said. “I knew I was able to put all my weight on it, move side to side OK, and still go out there and handle the workload of 90 to 110 pitches without a ton of fatigue. I didn’t know how long it was going to last.

"It was something that, in my mind, was going to happen at some point anyways. I’m fortunate that it happened when it did to actually have the month before I came down here. Now, [I have] six, seven weeks to prepare healthy for the season.”