Well, i get your point.....
But I do believe that Carrick's lesser game time has had more to do with recent United Managers not rating him high enough, than because he's not been fit or good enough.
Statistically, players who play in that position (defensive midfield, holding position) have had long careers, and have played well into their 30's.....So, I really can't see why Matic would be an exception, barring a terrible career threatening injury.
Players in same position such as Paul Scholes, Claude Makelele, Roy Keane, Patrick Viera, Xabi Alonso, Scott Parker, Mikel Arterta, Mathieu Flamini, Gareth Barry, etc..........................well into their 30's!