I agree with Phinz420. Ronnie would be a big loss if he gets hurt for an extended period of time. Sammy Morris is a good backup, but I don't see him as a guy who can start ten games at halfback and gain 80-100 yards a game.
Daunte would be second. I think despite what Joey Harrington says, he needs to regain his confidence. Being a part of that awful Detroit team would take a toll on anyone's psyche, especially when you're "the savior" of a team that won't save itself. I feel that he could be good if he'd come in for a few games, but a best case scenario would be him sitting on the sidelines and doing mop up duty with a winning team under a first class coaching staff.
Defensively, I'd say Zach and JT. Derrick Pope is a good backup, but teams won't gameplan around him. Losing JT would be horrendous. VERY few teams block JT one on one, so that frees up the LE in the 4-3 and we just don't have another linebacker with the pass rushing ability of Taylor.