Going through the scenarios:
#1 Rio retires this summer. Unlikely because it sounds as if he's going to get another contract but nevertheless, let's entertain the idea. He leaves, some might say we need an experienced center back to replace him. In comes Garay. Garay, Evans, Vidic, Smalling and Jones, with the exciting prospects of Keane and Thorpe in the background. How are Smalling, Jones, Keane and Thorpe supposed to develop if they don't start the games. I mean, in the big-matches, we'll probably toy with Garay/Vidic/Evans as they'd be our most experienced pairings. OK, Thorpe and Keane may go out on loan - but once they come back, they'll still be battling with a one-year older Garay, Evans, Smalling and Jones. Clearly one or even two of Smalling/Jones/Thorpe/Keane won't make it at United.
#2 Rio leaves next Summer. Our centrebacks have had a year extra of experience. Evans would be 26/27, Smalling 23/24 and Jones 20/21. Do we really need another player?
I just don't see the need. If Vidic AND Ferdinand are potential departures within the next year or so - then I agree. Even though I think Smalling is mature enough to stake a claim for a place, if we were against Madrid tomorrow, I wouldn't feel 100% confident going in with Evans-Smalling.
If we're really after Garay, then something interesting will have to happen.
Thus, I still think we're after someone that's based in Portugal.