PHILADELPHIA -- As the Mets filed into their clubhouse at Citizens Bank Park on Sunday, a familiar, no-doubt smiling face was already there, waiting to greet them one by one: Brandon Nimmo, back with the team after missing over three months.

Nimmo, who last played May 20 due to a bulging cervical disk and the effects of whiplash -- the results of an April 14 crash into the outfield wall in Atlanta -- was activated from the injured list before New York's 5-2 loss to the Phillies.

That means the Mets' most important September callup isn't really a callup at all. They're getting back a potential star outfielder just in time for the stretch run, with a playoff berth very much on the line. The Mets entered the season's final month four games out of an NL Wild Card spot. Nimmo is the first huge reinforcement, with another -- Robinson Canó, who appears nearly ready to return from a torn left hamstring suffered Aug. 4 -- maybe soon on the way.

"I'm excited," Nimmo said. "I've been watching the guys, the games on TV mostly. Just keeping up with them. It's nice to be here and be with the guys again -- I've really missed being around them.

"They've played amazing to get into this spot, and it really makes this last month fun. I'm glad they're enjoying it and really, really proud of the guys for fighting back to this spot."

Nimmo wasn't in Sunday's starting lineup, but manager Mickey Callaway called on the outfielder from the bench. Nimmo walked and scored as a pinch-hitter for Juan Lagares in the eighth inning, finishing the game in center field.

And Callaway expects Nimmo to be ready to start the opener of a crucial series against the Nationals on Monday in Washington. Cano could meet the Mets in D.C. the next day.

This is a moment Nimmo feared might not come in 2019. His return comes after an extended, frustrating rehab that included a month-long shutdown from all baseball activities when his injury was rediagnosed as nerve-related, rather than simple neck inflammation.

"I wasn't sure that this point would happen during this season, and I'm so glad that it has," Nimmo said. "After that first stint, you're starting to wonder, 'Am I going to come back this year without pain?' To be back without pain and a full range of motion, I'm extremely excited about it."

Before returning to the Mets, Nimmo played 10 rehab games with Class A Advanced St. Lucie and Triple-A Syracuse, going 8-for-35 with one homer, five RBIs and five walks. A previous rehab stint in early June ended when Nimmo was shut down when his neck pain recurred.

Behind Nimmo and Cano, the Mets will be waiting on one more big name: Jed Lowrie. The veteran infielder, who hasn't played in the Majors this season, is on rehab assignment at Triple-A, and Callaway said on Saturday the Mets are hoping Syracuse makes the playoffs so that Lowrie can continue to get game action with them.

If not, the Mets will have to decide where Lowrie goes from there -- Callaway said the Mets could set up sim games for Lowrie or "get creative," but they're waiting on the results of Syracuse's final two games before they commit to a plan. Lowrie will return to the Mets when he is fully ready.

"Players are always very careful about that. I think they're going to understand when that time comes," Callaway said. "He does feel like he's improving daily, so the dialogue has been good. But I think that when it comes down to it, he's going to understand that, 'Hey, I'm ready to go.'"

The Mets also made a trio of other September callups on Sunday, adding right-hander Tyler Bashlor and lefty Daniel Zamora to the bullpen and selecting the contract of infielder/outfielder Sam Haggerty. The 25-year-old Haggerty, whom Callaway called one of the fastest players in the organization, is the surprise name there. He'll serve mainly as a pinch-running specialist, with maybe some late-game situational duties mixed in.

Here's a look at some notable players from the Mets' system as September begins:

Arrival
There haven't been many bigger prospect arrivals in baseball than Pete Alonso, who broke camp with the Mets and went on set the franchise single-season home run record. Alonso could have been a September callup last season, but the Mets refrained in what was, at the time, an unpopular decision.

Breakout
Relatively unheralded heading into his first full professional season, left-hander Kevin Smith broke out to the tune of a 3.15 ERA over 23 starts with 130 strikeouts in 117 innings. Smith, who reached Double-A Binghamton this year, could be on the Mets' September callup radar this time next year. He is ranked as the club's No. 11 prospect by MLB Pipeline.

Something to prove
A strong summer could have put left-hander David Peterson in contention for a September callup, but the Mets' No. 7-rated prospect struggled early at Binghamton with a 6.88 ERA in April and never fully recovered. Still, the 2017 first-round Draft pick whiffed more than a batter per inning, providing hope that he can break out in his age-24 season.

Name to watch
Yet another left-hander, Thomas Szapucki, succeeded enough in his return from Tommy John surgery to make it to Binghamton by season's end. Although the Mets were careful with Szapucki's workload in his first year back from surgery, they plan to send their No. 12 prospect to the Arizona Fall League later this month. Next season will be critical to Szapucki's development as he looks to tack innings onto his ledger in the upper Minors.