Baez, Kevin Pillar and Francisco Lindor all made the hand movements during their win over the Washington Nationals on Sunday ... with Baez later explaining it's all 'cause NY fans have booed them during their recent slump.

"They gotta be better," Baez said of Mets fans after saying he does hear the jeers after poor at-bats at the plate.

"I play for the fans and I love the fans, but if they're going to do that, they're just putting more pressure on the team and that's not what we want."

"We’re not machines," Baez added. "We’re going to struggle. We’re going to struggle seven times out of 10"

Not long after Baez finished giving those comments ... Alderson made it known he would NOT accept the gesture from the team's new superstar or anyone else on the squad going forward.

"These comments, and any gestures by him or other players with a similar intent, are totally unacceptable and will not be tolerated," Alderson said.

"Mets fans are understandably frustrated over the team’s recent performance. The players and the organization are equally frustrated, but fans at Citi Field have every right to express their own disappointment. Booing is every fan’s right."

73-year-old Alderson added he will be meeting with Baez and the rest of the team and staff to make sure they get the message.

"The Mets will not tolerate any player gesture that is unprofessional in its meaning or is directed in a negative way toward our fans," Alderson said, adding, "Mets fans are loyal, passionate, knowledgeable and more than willing to express themselves."

"We love them for every one of these qualities."