Mets exec Sandy Alderson is furious with Javy Baez and other players for giving thumbs down gestures to fans this weekend ... with the team prez calling the moves "unprofessional."
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.