Victor Wembanyama got the opposite of a warm welcome in his first game at Madison Square Garden ... 'cause a bunch of ruthless Knicks fans made it clear they're not believing the hype surrounding the hoops phenom -- chanting "overrated" in the middle of the game!!
Of course, the San Antonio Spurs' No. 1 overall pick ruffled some feathers with the home crowd prior to Wednesday's matchup with New York ... when he said the Mecca wasn't as big as he expected.
Knickerbocker backers promised to give the 19-year-old hell as a result of his comments .