Warren Moon fully believes that -- with the Hall of Famer telling TMZ Sports he thinks the 3-4 Bucs could turn everything around if Gronk came back this year.

Moon tells us he got the opinion based on everything he's seen out of Tom Brady on the field through seven games. He says he's witnessed the GOAT struggling in the pocket ... and missing an old reliable on important downs.

"He's been a security blanket for [Brady] for so long," Moon said of Gronkowski. "Whenever there's a key play that needs to be made, especially down in the redzone or whatever -- and they had a tough time scoring in the redzone last week -- that's the guy he would usually go to, Rob Gronkowski."

Brady's numbers haven't been horrible so far in 2022 -- he's got 8 passing TDs to just 1 interception -- but something does appear to be off. He only has one multi-TD game this year, and just two performances of over 300 passing yards.

Moon clearly believes Gronkowski could change all of that, and even help out the locker room with his "infectious personality" ... but there's just one problem -- he's not sure if the tight end is even considering coming out of retirement.

Gronk has said repeatedly over the last few months he's done, although his agent, Drew Rosenhaus, has claimed it wouldn't surprise him to see a comeback.