It all went down just prior to the Eagles' "Monday Night Football" game against the Atlanta Falcons at Lincoln Financial Field ... where Kelce was trying to rowdy up some fans at a tailgate outside of the stadium.

The former Birds offensive lineman hopped on the mic and gave an epic pump-up speech ... and to put an exclamation point on things -- he brought Rob Dunphy out for a fun celebration.

He told the topless fan -- who's gained fame over the years for his wild Philadelphia tattoos -- to get down in a three-point stance ... and then meet him in the air for a touching of tummies.

But, check out the video ... as Dunphy landed -- he tweaked his leg.

He tried to walk it off, but a short time later, he wrote on X, "F*** my whole knee up." Thankfully, the ailment doesn't seem too bad -- as he added, "So worth it lmao."

Kelce, meanwhile, seemed oblivious to the whole situation ... and went on to have a helluva night in his new broadcasting role.

Not only did he work ESPN's pregame show smoothly, but he joined Joe Buck and Troy Aikman in the broadcast booth for a large portion of the second half -- and received a ton of praise for his analysis of the game.