Doc Rivers -- Disgusted With Paul Pierce's 1st Pitch

He hasn't played 1 minute for the Clippers yet ... but Paul Pierce has already found a way to DISAPPOINT his new coach -- when he CHOKED on the mound at the Dodgers game.

FYI -- Pierce had the honor of throwing out the 1st pitch at Dodgers Stadium earlier this week ... but failed ... and got showered with boos from the crowd.

So, when we saw Doc Rivers in L.A. yesterday, we showed him the footage for the first time to get his candid reaction -- and it was hilarious.

"On no. That is awful."

Check out the clip ... Rivers does offer a pretty good excuse for Pierce ... and then reveals an embarrassing baseball secret about himself.