What if Thor, being a priority target and all, is what the villain focused a lot of power on, until Thor becomes incapacitated. Villain then proceeds to screw everyone else up, and Steve loses his shield.
Villain doesn't consider Steve to be any more of a threat, and moves on to the others, but Steve, always the fighter, decides to at least try the hammer that happens to be in range, and surprise surprise, it accepts him with no problem, especially given the circumstances.
Villain has his back turned, but notices the burst of energy behind him. Before he can turn around and attack, Cap lays down a powerful blow that turns the tables and allows other capable Avengers to put on pressure until Thor gets back on his feet. Steve returns the hammer to its master and the two of them go in for the final strike as brothers in arms.
At least that's how I'd like to see it. Steve gets a his moment, but it's not the deus ex machina that's the sole reason the villain is defeated. Instead it's just a bad ass opener that allows the final assault from everyone else to happen.