Washington Nationals reliever Shawn Kelley was frustrated over the “mixed messages” he received from the umpiring crew — not over being summoned to pitch in a blowout game — when he slammed his glove to the ground in anger during Tuesday’s 25-4 victory over the New York Mets, his agent told ESPN.

The incident prompted the Nationals to designate Kelley for assignment Wednesday morning, and the team now has seven days to trade, release or place him on irrevocable outright waivers.

General manager Mike Rizzo called it a “selfish act” on Kelley’s part, and manager Dave Martinez said it was “disrespectful to the organization.”

Agent Mike McCann said Kelley had no issues with pitching in a game that Washington led 25-1 at the time. Kelley was fresh from four days’ rest and began warming up in the bullpen before the phone rang to tell him he would be called upon to pitch the ninth inning, his agent said.

McCann said Kelley grew frustrated when home plate umpire Adrian Johnson implored him to work more quickly, while second base umpire Tripp Gibson simultaneously told him to slow down or be called for a balk.

Kelley looked into the dugout for help because he didn’t want to argue further with the umpires, be ejected from the game and force the Nationals to use another reliever, the agent said.

After giving up a two-run homer to Austin Jackson, Kelley slammed his glove to the mound in anger. Three batters later, he retired Amed Rosario on a groundout to end the game.

“I thought the act that he portrayed on the field last night was disrespectful to the name on the front of the jersey, the organization, specifically Davey Martinez,” Rizzo said before Wednesday’s game against the Mets. “You’re either in or you’re in the way. And I thought he was in the way.”

When informed of Rizzo’s comments, McCann replied, “That’s his opinion.”

Kelley apologized for his actions after the game, and declined to respond to Rizzo’s comments through his agent.

“He knows throwing the glove was wrong,” McCann said. “It was unfortunate, but it had absolutely nothing to do with frustration with the Nationals. He wishes the Nationals and his teammates the best of luck the rest of the way.”

Kelley, 34, is 26-23 with a 3.73 ERA over 10 MLB seasons as a reliever with the Mariners, Yankees, Padres and Nationals. He signed a three-year, $15 million deal with Washington in December 2015.

The Nationals entered Wednesday at 53-53 — 5.5 games behind first-place Philadelphia in the National League East. Amid a flurry of trade rumors before Tuesday’s non-waiver deadline, Rizzo made only one deal — sending reliever Brandon Kintzler to the Chicago Cubs for minor-league pitcher Jhon Romero.