Sounds to me like taking iron supplement without needing it, which is more harmful than it does good. I would take my chances with the defenders available. There are 5 fullbacks and five cbs. No need for more.
Oh, an the "it's not your money" argument - seriously?
Every top club needs 2 players for every position. So: 2 fullback positions = 4 fullbacks, 2 centre-back positions = 4 centre-backs. We are one over that quota for those positions by havin 5 each.
Take into the equasion that Rio has on-going fitness struggles with a specially designed training programme to meet those needs, he cant play 2 games a week on more than 2 occasions per month - if even 2.
Vida has missed something like nearly 2 whole seasons out of 3 & he's not on tour because of sciatica & he cant play 2 games a week on more than 2 occasions per month either - maybe just 1.
Vida & Rio cant be risked to play any more than 3/4 games per month, to make sure they're available for the big games both domestically & in Europe. They'll continue to be rested when its possible, especially against the mid-to-lower half league teams at home, but away - that might be a different story depending on the form of the so-called lesser sides.
Worst-case scenario is that both Rio & Vida struggle with injury's for large portions of the campaign, possibly leavin only 3 fit centre-halfs available at certain periods of the season. Evans is also injury prone & Smalling is still recovering from last seasons injury & operation, so we dont know how he's gonna be after that to be honest. Jones is probably our only fit defender all the time & thats him possibly covering right-back & centre-midfield too. I still think he needs more games at CB to be truly trusted in top games, especially in Europe & he'll need to get those games of course - Smalling the same, who can also cover at Right-back.
So out of 5 CB's, 3 of them are injury-prone, 1 of them is coming back to fitness after a lengthy lay-off with injury & operation recovery, so that could be problematic. Which leave us with pretty much 1 reliably fit CB in Jones. I dont think that brings great security whatsoever & given the fact that Rio will probably retire after this season - bringin Garay or another into the club, will enable an easier transition (as he'd be bedded in nicely by then) to compete for Rio's position when he's gone!
We've been ravaged defensively for the past 3 season & we cant count on what we have to stay fit. Garay or another, would make sense to bring in for the reasons i've mentioned!