I think Lampard or Rooney both keep us up, and then go on to have terrible starts next year and are gone by this time next season.
Lampard will do his one trick of coming in and injecting energy with youth players. Branthwaite, Onyango, Gordon, Dobbin and Simms see more minutes and we get a bounce, a few good results (and some pats on the head for tigerish performances in losing to big teams). Then, the players settle into the team, energy levels drop a bit and lack of actual tactical nous comes to the fore.
Rooney almost identical, except the energy comes more from the chest-thumping "WHO ARE WE?" mentality he'll bring. We'll grind out some good results and maybe even take a big scalp or two along the way.
Kovac has the best CV by a mile. If he or Marsch are remotely interested we should be going after them. We won't, though.
Long story short, welcome back, Big Sam.