You don't know football if you think Kirk Cousins ain't worth his $84 MILLION deal ... so says his teammate Laquon Treadwell who tells TMZ Sports the QB is "a baller."

Of course, Cousins had a pretty rough first season in Minnesota after inking a mega-contract ... losing a must-win game to the Bears that kept the Vikes outta the playoffs.

But, 23-year-old Treadwell -- the Vikes' 1st-round pick in the 2016 Draft -- says just because the first season didn't go according to plan doesn't mean the QB wasn't worth the money Minnesota paid him.

"That's a lie, man," Treadwell told us when we got him out at LAX this week ... "He a baller."