New York Mets starting pitcher Zack Wheeler reacts during the...

New York Mets starting pitcher Zack Wheeler reacts during the first inning against the Philadelphia Phillies in an MLB baseball game at Citi Field on Sunday, July 7, 2019. Credit: Kathleen Malone-Van Dyke

SAN FRANCISCO — Zack Wheeler appears to be closing in on a return.

He threw a bullpen session Sunday morning at Oracle Park, his first since being sidelined with a right shoulder impingement. The Mets will wait to see how he feels Monday before deciding on a next step: back into the rotation or a simulated game.

“He came out of it well,” Mickey Callaway said. “We have to figure out when to plug him in or if he needs to face hitters before we plug him in.”

Jason Vargas will start Tuesday against the Padres, tentatively followed by Noah Syndergaard and Jacob deGrom. If the Mets started Wheeler on Wednesday, it would allow him up to two starts before the July 31 trade deadline. If not, he likely would have only one game to prove to teams interested in acquiring him that he is healthy.   

More Diaz decisions

Another evolution in the Mets’ rules that guide their use of Edwin Diaz: Callaway said he would have used Diaz for the bottom of the 11th — a full second inning — had the Mets gone ahead in the top of the inning Sunday.

The Mets have been adamant that Diaz will not get more than four outs, upped already from the three-out maximum to begin the year. On Sunday, Callaway cited the low-stress, 11-pitch 10th and a day off Monday as reasons to push Diaz past four outs.

“He was going to go back out if we took the lead,” Callaway said. “That would’ve been a big win.”

The Mets didn’t score in the 11th, so Jeurys Familia replaced Diaz. They lost, 3-2, in 12 innings.   

Extra bases

Justin Wilson tossed a perfect inning Sunday and is up to 6 1⁄3 scoreless since returning from the injured list July 2 .  .  .   The Mets are teaming with New York Blood Center to host a blood drive from 11 a.m.-6 p.m. Monday at Citi Field. Donors will receive a voucher good for a pair of tickets to a select Mets home game.