PITTSBURGH — Pete Alonso’s second-half slump finally screamed loudly enough to warrant a “mental day” for the Mets first baseman Sunday.

The rookie slugger is in an 11-for-73 (.151) skid since the All-Star break with four homers and 30 strikeouts. Todd Frazier shifted to first base for his initial start of the season at the position. Alonso entered as a defensive replacement in the seventh inning of the Mets’ 13-2 victory over the Pirates, but didn’t have an at-bat.

“Petey is getting a mental day off today and that’s a good thing for him,” hitting coach Chili Davis said. “Just relax a little bit. He didn’t get an All-Star break. He was mentally involved in the All-Star Game. Just giving him a little break, I think it will help.”

Alonso won the Home Run Derby in Cleveland and then participated in the next day’s All-Star Game. Manager Mickey Callaway also gave Alonso a mental day two weeks ago in San Francisco. Alonso responded by coming off the bench to hit a three-run homer.

“[Pitchers] are trying not to give him a whole lot to hit and he’s really been working at being selective at the plate, but it’s kind of an in-between thing,” Davis said. “You don’t want to be too selective and not swing at your pitch. I think sometimes you try to be selective and then you start taking pitches you should hit.”

Alonso was clean shaven after the game, having shed the beard he’s worn for much of the season.

“I stopped hitting with the beard, so I decided to shave it,” Alonso said.

Jacob deGrom will pitch Game 1 of Monday’s doubleheader against the Marlins and Walker Lockett will be recalled from Triple-A Syracuse to start the nightcap. Lockett earned his first major league win in his last appearance for the Mets on July 20 in San Francisco. In that game, Lockett pitched five innings and allowed one run.

Jacob Rhame was placed on the injured list before the game with right elbow discomfort and lefty reliever Donnie Hart was called up from Syracuse. Hart was claimed from the Brewers a day earlier. Hart pitched a scoreless eighth inning.