Let's not get too far ahead of ourselves here, either. There are no guarantees where Manning is concerned. Sure, his workouts have looked good. Sure, he is the best quarterback of this generation.
However, he's on the wrong side of 30, coming off more than a year of not playing in an actual game and he's coming off an injury that's about as scary as they come.
Let's not forget he didn't have a good, fast recovery either. He had to have extra surgery done. He missed more time than anyone expected.
The Titans have shot themselves in the foot before (Vince Young, anyone?) at the quarterback position, but they had it right last year with Hasselbeck tutoring Locker and the team responding to that steady leadership.
Manning will be a leader. That much is obvious. However, do the Titans want to risk what they've built the last year or so? I'm not sure it makes sense.