Dustin 'Screech' Diamond Fighting 'Bagel Guy' in Celeb Boxing, Replacing Lenny Dykstra

Lenny Dykstra is out. Screech is in.

TMZ Sports has learned Dustin Diamond has agreed to replace Lenny Dykstra and fight Bagel Guy Chris Morgan in a Celebrity Boxing match this weekend in New Jersey!

As we previously reported, Dykstra pulled out of the event this week because he's focusing on an unrelated lawsuit.

So, Celeb Boxing promoter Damon Feldman reached out to the former "Saved By The Bell" star and we're told 42-year-old Diamond agreed almost immediately!

Now, the fight is on ... the two will clash at the Showboat Casino in Atlantic City on Sept. 28.

Screech is 6'0" tall and 45-year-old Morgan is listed at 5'0".

As for Bagel Guy, Chris Morgan has been training his ass off ... working with UFC stars like Chris Weidman and Al Iaquinta.

Unclear if Dustin has been training but he's been involved with celeb boxing in the past -- fighting Ron Palillo -- aka Horshack from "Welcome Back, Kotter" back in 2002. Dustin won the fight after knocking Palillo down a few times.

Ya gotta think the experience will give Screech the advantage.