Seemingly every game Jose Quintana starts allows the Mets to think about what could have been.

Sunday’s outing made Quintana appreciate what has been.

Quintana, whose season did not begin until July because of bone graft surgery that was prompted by a rib fracture, has made a strong return in his 12th big-league season.

Yet another quality start in the Mets’ 8-4 win over the Reds at Citi Field represented the 300th career start of Quintana’s career.

“It means a lot. [I’m] healthy. It’s a special number,” Quintana said after he limited Cincinnati to two runs in 6 ²/₃ innings. “You do it for a long [time]. I always try to do the best I can do.”

The lefty, who is on his seventh team, is one of 10 active pitchers on a major league roster to reach 300 career starts, joining Clayton Kershaw and Wade Miley as only southpaws on that list.

Quintana has never been a fireballer and has continued to excel with a deep pitch mix, inducing weak contact and throwing strikes.

He allowed eight hits but just one walk in lowering his ERA to 3.02 in 11 games.

The 34-year-old has completed at least five innings in all 11 of those games. He seems to be getting stronger, allowing just five runs in his past four starts, a span of 24 ²/₃ innings (1.82 ERA).

“He’s the type of guy that other pitchers are drawn to,” manager Buck Showalter said of Quintana, who is signed through next season. “He’s got a nice, quiet, no-ego [demeanor]. … When he speaks, the more inexperienced guys listen. And he’s a pretty good pitcher.”

As Quintana tried to take stock of what he has been able to do on the mound, Showalter remembered the injury that forced Quintana to make just two starts before August.By the time he returned, the Mets’ season was over.

“It’s kind of bittersweet,” Showalter said. “Every time I see him pitch, I think about the two-thirds that he wasn’t here.”

Jeff McNeil started his first career game in center field, completing the game without incident. The bulk of the utility man’s work has come at second base this year, but moving to right and left field has opened time for infielders from Luis Guillorme to Ronny Mauricio, who played second Sunday.

McNeil, who had played two innings at the position last month, moved to center in part to give Brandon Nimmo a half-day off as DH.

“I think it’s just a reminder of are you willing to do it, and he’s definitely willing, and he likes it,” Showalter said of McNeil. “They give that utility award, and sometimes I think they give it to the guy that plays the most positions instead of the one who’s the best at multiple positions. That award should be given to someone that’s the best at multiple positions.”

McNeil has a case, having played solid defense wherever he has been shifted.Showalter said McNeil has not been “bugging” him about playing center, but McNeil has offered “a reminder that he can do it and would like to do it.”

Brett Baty (groin tightness) was out of the lineup for a fourth straight game. Showalter hopes the rookie can be ready to play Monday in Miami but acknowledged the groin is healing “a little slower than we hoped.”

Baty is expected to get treatment and go through testing Monday.

Dennis D’Agostino, a Mets public relations assistant in the 1980s and a longtime head of public relations with the Knicks for whom he was also the team’s historian, died Saturday night of a heart attack, the Mets announced. He was 66.