Tristan Thompson has his legal guns putting the heat back on the Instagram model claiming she has salacious texts from him ... telling Sydney Chase to put up or shut up.
The Celtics star -- who's with Khloe Kardashian -- had his attorney, Marty Singer, fire off a legal threat to Chase ... who claims she had sex with Tristan after he got back with Khloe.
In the letter, obtained by TMZ, Singer says Chase is refusing to prove the existence of the alleged texts and it can only lead to one "inescapable conclusion .