CHICAGO — The diagnosis was delayed and the recovery long, but the rehab assignment was lightning quick.

Jake Cronenworth played three games for Triple-A El Paso over the weekend before being activated off the concussion injured list and rejoining the Padres on Monday.

“Relief,” Cronenworth said Monday afternoon. “Long process, but super happy to be back.”

Cronenworth, who at times early in the process could not keep his balance when leaning over, spent the past eight weeks letting his brain recover and then retraining his eyes to track pitches.

He never thought he wouldn’t play again. Not exactly.

“You just don’t know,” he said.

That is how scary and slow and full of setbacks the recovery was.

“It has been tough,” Cronenworth said. “I had good days, I had some really, really bad days consecutively where I felt like I was never getting better. I’d get a glimmer of hope every once in a while and just kept fighting through. It was tough. But great support group, and that was awesome.”

Cronenworth started hitting against minor-leaguers at the Padres’ complex in Arizona a little over a week ago and on Monday was in the starting lineup playing second base and batting ninth for the Padres’ series opener against the Cubs.

“Great to have Croney back,” Padres manager Craig Stammen said. “We missed him, definitely. A guy who has been a part of our team for a while. Someone that is just Steady Eddie, professional at-bats all the time and then a great second baseman. He’s just value added when we get him back.”

Cronenworth was batting .144/.272/.196 when he went on the IL. He spent the first 19 games of the season as one of the unluckiest hitters in baseball, with a .167 batting average on balls in play that  ranked seventh lowest among qualifying MLB batters. He was struck on the side of the head by a 97 mph fastball from Angels pitcher Yusei Kikuchi on April 18 and played the next 12 games with a concussion before reporting symptoms to the Padres on May 5.

He said then he had been having difficulty with depth perception and judging the location of pitches.

“I was going home confused every night, looking at my at-bats and wondering why I’m swinging at certain pitches — and two, three times in an at-bat — and taking certain pitches that I usually don’t take or swinging at pitches that I usually take,” he said Monday. “That was the thing that was super confusing, because I felt like my swing was in a really good place.”

Croneworth said he lost eight pounds between the time he was hit and when he went on the injured list. He was sensitive to light and tired all the time. He has seen pictures taken during that time and notes the bags below his eyes and his pale skin.

As he noted when he went on the IL, he said Monday continued to play because he figured the grind of the season had gotten to him earlier because the team had a long road trip that included seven days at high altitude.

“I think it was just I didn’t notice it, because you never feel good during the season,” he said. “You have times where you feel good, times where you feel bad. We were in Colorado and Mexico City for a week. I don’t think anybody in here felt great after we got back. I’m not gonna complain about it.”

Once he was shut down, it was nearly a month before he was doing baseball activities in earnest.

Virtually every step of his recovery — playing catch, hitting off a team, hitting soft toss, hitting off a machine — was laborious.

“I remember hitting off the tee for the first time, or playing catch for the first time,” he said. “I literally felt like I was drunk.”

He then held up his hand and simulated a wave to illustrate what the ball looked like as it was coming toward him.

“The next day it got a little bit better and the next day a little bit better,” he said. “And then, like a week in, it was normal.”

Cronenworth has visited an ophthalmologist several times and now begins his days with eye exercises.

“It was so prolonged because of my vision,” he said. “That was the one thing that really wasn’t getting better at all, and it’s the one thing that I really need. That was the biggest concern to me was my eyes, for sure.”