Delph and Fernado's midfield role cab easily be covered by Garcia. Granted both currently show a little more defensive bite than Garcia, but I think his ball possession potential nullifies their advantage unless we a man down and are simply playing survive the last 20 or something like that.
Pep's said quite a few times he sees Fernando basically as the muscle. Someone to come in and throw his weight around, break up play and make tackles. Sort of how he used Keita at Barcelona I guess.
Garcia can't replace him in the squad, they have totally different roles.
Fernando and Delph shouldn't both stay though, they're passengers basically, and I'm not even sure what Delph's niche in the squad is.