Bill Belichick and Patriots Finally Taste Defeat at Celtics Game

Bill Belichick, some Patriots players and the city of Boston all felt the same, weird abnormal feeling going through their bodies Thursday night ... LOSING!!!!

... and they can thank the Celtics.

The Pats coach -- sporting a snazzy checkered jacket -- sat up front at the TD Garden with his girlfriend, Linda Holliday, to watch the Celts take on the L.A. Lakers.

Sony Michel, Trey Flowers and Rex Burkhead were also at the game to support their Boston brethren.

Remember, the last time a Boston team took on an L.A. team ... it was boring as hell but also good for Beantown.

This time, it was the exact opposite -- a buzzer-beating thriller that ended in DEFEAT for the guys in green!

And, the guy who hit the game-winning dagger ... former Celtics star Rajon Rondo!

BTW, it's the first loss B.B. and the gang have felt since mid-December when they took an L to the Steelers in Week 15.

Obviously, Belichick don't care ... he's still got 6 rings (as a head coach) plus 2 more from when he was an assistant.