All good points. I am not so much concerned about Mellen being out because we don't have another Alpha Defender (as stated I think Kennedy can be that guy, he'll have to be that guy next year), it's the fact that the defense will be playing without a current starter, and that has got to impact the performance when he comes back. This team really needs to gel and be a cohesive unit. Maybe they'll do that when he's out, but they'll still have to reintegrate him when he is healthy.
Mellen is a very good defender but he's also very unique, in that he usually does very well against the smaller/shiftier attackmen that other teams with bigger stronger defenders can't keep up with. If he is closer to the goal against a typical X attackman, he's excellent. His feet are incredible. But, I don't think he has the best end to end speed - I think Brown was successful against him because he brought him out away from the goal, Brown was a midfielder last year after all. In the one instance where Mellen guarded a middie (two years ago against Navy) it did not go well. Mellen is almost like a secret weapon, because so few teams have defenders like him with his specific talents.
Kennedy on the other hand has a very good balance of speed and aggressiveness. I agree he'll do well against players like Epstein and Nichtern (Mellen did very well against both last year), but part of me wonders how he would do against Sowers or Ament. Good thing neither are on the schedule, but it's worth thinking about.