If Pat Tillman ever gets honored at the Pro Football Hall of Fame, Jake Plummer will be the first one there for the ceremony ... although he tells TMZ Sports if it doesn't happen, he knows the hero's legacy will go on regardless.

Plummer and Tillman were good pals before the Army ranger's tragic death in 2004 -- the two played together at Arizona State and then once more with the Arizona Cardinals -- and when we asked if it was time to include the former defensive back in Canton, Plummer said he'd be for it.

After all, despite playing in just 60 games in his pro career, there might not be a more universally respected person in the history of the NFL.

"If it was about impact," the ex-Cards quarterback said, "then, hell yeah, he should be in there."

But, Plummer said he's not going to sweat it if Tillman never gets a bust someday... telling us he's not even sure if Pat would've wanted the honor -- because he was just that humble.

Plus, Plummer can take solace in knowing Pat's already kind of in there -- explaining to us someone snuck a Tillman card into a mural that's already up in the museum.

Monday, if you weren't aware, marks the 20th anniversary since Tillman was killed while serving his country ... and Plummer has already spent a ton of time this month remembering his good friend. He participated in the Pat's Run race in Tempe with some 30,000 others on April 13 -- and, in between, he tells us Pat's been all over his mind too.