CHICAGO -- When the visitors' clubhouse in Wrigley Field opens on Friday and Saturday, A.J. Burnett is nowhere to be found. He’s skipping all of that, the usual routine of lingering in that cramped, claustrophobic, inadequate space tucked under the lower deck on the first-base side. Instead, he’s where anybody in their right mind should be in Wrigley Field: outside, on the grass in center field, out in that late September sun on mornings with those first hints of the cold to come.

And that’s because he’s out there playing Frisbee with a teammate. At Wrigley Field.

Like an astronaut golfing on the moon, it’s the kind of thing you do because you can, because you had long since earned the right to be in a place most people never reach. And when else are you going to get the chance? But it’s also something that, after Sunday, he’s not going to get to do again, because after pitching against Cy Young contender Jake Arrieta on Sunday night, A.J. Burnett probably will never see Wrigley Field as a ballplayer again. As is, he might only pitch one more game -- ever. That's because Burnett has said that he’s going to retire after this season, leaving the game at his own speed after 17 seasons.

Given his last chances, A.J. Burnett is doing things he's never going to get to do again on the field. Charles LeClaire/USA TODAY Sport

“Not many of us walk out of our own volition,” Pirates manager Clint Hurdle said Saturday. “But A.J. has earned that right, earned that privilege. For him to have this opportunity, it’s special and significant. We’ve been collecting baseballs all year, and he’s going to have that start at home, which I think is going to be unique. They had the Batman lights on the city! It’s cool stuff.”

Pittsburgh literally did light up a Bat Signal on Sept. 10 when Burnett, a huge Batman fan, returned from the DL. His 2015 season, his final season, a year which had been his best yet with a 2.11 ERA that earned him his first and only All-Star invitation, was at risk when he was shelved at the end of July with right elbow inflammation around his right flexor tendon. But he’s come back and demonstrated he still has something left and can still be counted on as one of the rotation regulars the Pirates will rely on into October. But for him to get even one more start beyond next week’s postseason tuneup in front of the fans in PNC Park will be up to the Pirates to make it to a division series, to not just earn a wild card but win it. Or the division.

Hurdle was being coy as he went over his postseason rotation plans, noting, “I have not looked beyond today. We’ve lined [Gerrit] Cole up to pitch the wild card; if there’s a play-in game, Cole would pitch that and [Francisco] Liriano would pitch the wild-card game. If none of that happens, Cole will be lined up for the first game of the division series. We have six [starters] right now; it’s a five-game series and you don’t need five. But we can have that conversation now, based on performance.”

But in Burnett’s three starts since his return to action, Hurdle has already liked what he’s seen, observing, “The first game out, he’d been away so long, his fastball was elevated, he gave up three quick runs, and then it was four zeroes; fastball angled down, mixed in some breaking balls, threw a few changeups. The next time out, against the Cubs, the one guy he wasn’t able to locate the ball to was [Kris] Bryant. They got some guys on base, but his ability to pitch from the stretch was fantastic. He spun the ball as well as we’ve seen him at his best as far as his breaking ball; his fastball command wasn’t where it can be, but the velocity was back and was firm.

“I’ll be excited to watch him pitch [Sunday], to see where that takes us."

He’s not the only one. In the clubhouse, the Pirates are ecstatic that Burnett is back in action and see his return to action as the reunification of their big front three in the rotation: Cole, Liriano -- and Burnett.

“He’s gotten better every time out. I’m looking forward to tomorrow,” Cole said with a grin after Saturday’s win. “We have our three big guys going in this series; we’re going to need to take care of St. Louis and then hopefully we can put ourselves in a position to play deep into October. There’s no better time for him to be hitting his stride than now.”

Pirates players have valued what having Burnett as a teammate has taught them about pitching, because that’s what Burnett, once a live-armed hurler, has become: a pitcher and a possibility model.

“I’ve been a hitting coach where I’ve had to put game plans together against A.J. back when he was a Marlin,” Hurdle recalled. “I’ve seen the complete evolution, watching this man pitch. He was a thrower, and now he’s flat-out become a pitcher, a very cerebral pitcher. It’s not coincidence; it was a choice. I think it took courage for him, but I also think it also took common sense. He’s been able to be an honest self-evaluator: 'Here’s what I’ve got, here’s how I can best use it.'"

A.J. Burnett and teammates celebrate as the Pirates rock another opponent. Jared Wickerham/Getty Images

“I’ve learned from him that you can be a two-pitch starting pitcher,” said righty Charlie Morton, one of the starters taken down a notch in the rotation with Burnett back. “There have been times when you might get inconsistent and start thinking you need to add a new pitch. But I’ve learned from A.J. what you can do, having confidence in just two pitches.

“His patterns are very simple, our patterns are similar, but the execution is different. Ninety percent of the two-seam fastballs that he throws to lefties, maybe more, are going to be on the inside corner. And then he’s going to throw a back-foot breaking ball and mix in a changeup every now and again. Righties, he’s going to stay 90 percent away, bounce that breaking ball and mix in a changeup now and again. It sounds easy, but it just goes to show you how you can succeed when you simplify things.”

He was a two-pitch pitcher until recently, because in his third turn since coming back from the DL, Burnett mixed things up a bit. Rather than keep it simple, he mixed in a changeup more than he has at any time in any game since 2011, all to conjure up a win.

“The last game, at Coors, this is an older man with experience finding a different way to cut up a lineup -- he threw 21 changeups!” Hurdle enthused. “I think the first year he was here, he might not have thrown 21 changeups the whole year he was here, the entire time.”

It’s that willingness to mix things up that might have people guessing that the Burnett they knew might be somebody with a last trick or two up his sleeve.

“We should just expect the unexpected from him; I do,” pitching coach Ray Searage says with a chuckle. “I’m just like, 'What have you got for me today, kid?' He’ll figure out a way to keep the team in the game. Now all of a sudden he’s got three pitches -- really four with the fastball to both sides of the plate. Now they can’t sit on pitch or one location.”

Which leaves the man doing everything for a last time, nevertheless trying to do something different in the few opportunities left to him before he calls it a career, whether that’s after one more start or five or six.

“The game takes care of those who take care of the game, and A.J. has been on both sides of that coin and has figured out how much better it is to take care of the game,” Hurdle said, remembering the wild child on the Marlins he once opposed, and the man on the edge of exit he manages now.

“I think he believes in that, that he’s going to pour his heart and soul into every start,” Searage said. “And whenever it ends, it ends.”

Christina Kahrl writes about MLB for ESPN. You can follow her on Twitter.