Johnny Manziel -- Prepping To Throw 1st Pitch ... I Don't Wanna Pull a '50 Cent'

8:00 PM PT -- No throwin' for Johnny ... officials canceled the 1st pitch due to the rain delay. Sucks.

Johnny Manziel is currently warming up his arm to throw out the first pitch at the Cleveland Indians game tonight ... in an effort to not embarrass himself on the mound, a la 50 Cent.

Manziel was seen throwing pitches in a practice with fellow Cleveland Browns rookie Justin Gilbert (the #8 overall pick).

Manziel's form looked good in the warm-up pics -- but then again, so did Fiddys.

The game is currently on rain delay -- if his arm cools down, it could spell trouble for the Heisman Trophy winner.

Then again, Manziel played baseball growing up ... and throws for a frickin' living.