MIAMI -- Telling Brandon Nimmo he must go to the Minors, Mickey Callaway said, was one of the most difficult conversations he has had with a player in all his years as a coach and manager. The fact that Nimmo took it as well as he did could not have helped Callaway's emotions.
But with Nimmo confined to the bench for now and the Mets leaning heavily on their relievers in recent games, they wanted to shift to an eight-man bullpen. That meant a demotion for Nimmo and a promotion for 16th-ranked prospect Corey Oswalt, whom the Mets recalled from Triple-A Las Vegas before Tuesday's 8-6 victory over the Marlins. Following the game, the Mets optioned him as the corresponding roster move for Zack Wheeler, Wednesday's starter.
"A tough break for Nimmo," Callaway said. "He deserves to be here. He handled the news unbelievably -- what a guy. He's a winning player and he's going to help us for a long, long time. So that's tough. But we felt like it was necessary."

The Mets' Opening Day center fielder, Nimmo saw his role reduce drastically when Michael Conforto came off the disabled list last Thursday. He made just three plate appearances in four games, with Conforto starting all but one game in center.
The Mets considered it more valuable to fortify their bullpen with Oswalt, 24, who allowed three runs in four innings last Thursday in his first career start at the Triple-A level. The Mets' organizational pitcher of the year in 2017, Oswalt went 12-5 with a 2.28 ERA at Double-A Binghamton to earn a spot on the Mets' 40-man roster after the season. He did not appear on Tuesday to make his Major League debut.

Barring an injury on the Mets' 25-man roster, Nimmo must stay in the Minors for at least 10 days. He was batting .333 with a 1.044 OPS in 15 plate appearances, which came on the heels of a strong September and big league Spring Training.
"He wasn't getting to play as much as he probably needs to play," Callaway said. "This kid's going to be an everyday player. Anytime we can get him as many at-bats as possible is probably to his benefit."
For Nimmo, Conforto, Yoenis Cespedes and Jay Bruce all to start, Bruce would need to move to first base -- something the Mets were unwilling to explore during Spring Training. But Bruce began taking reps at the position for the first time on Tuesday, keeping himself versatile in case the Mets do ask him to do that later this summer.

Wheels up
Joining the Mets in Miami in advance of his Wednesday assignment, Wheeler said his successful debut at Triple-A -- six strikeouts in five innings of one-run ball -- was due in part to his increased comfort with a new delivery. Wheeler has been working since spring on keeping his arm higher while throwing to decrease the stress on it.
"It's hard because you've been doing something for so long," Wheeler said. "It's muscle memory to sort of just drop back down."

With Jason Vargas recovering slowly from surgery to remove a broken bone from his glove hand, Wheeler should have a chance to make at least two starts in the big leagues -- and possibly more, if he thrives. A favorite to make the rotation when Vargas broke his hand, Wheeler lost that opportunity when he went 0-1 with an 8.10 ERA in five Grapefruit League outings. He has made just 17 starts, with a 5.21 ERA, since undergoing Tommy John surgery in March 2015.
"Stuff happens," Wheeler said. "I'm grateful to be up here and hopefully I can make a little statement."
Tuning in 
The Mets are as hot on the airwaves as they are on the field. Their win Monday over the Marlins drew a 4.50 household rating, SNY's second-highest-rated broadcast since the 2015 postseason chase.