You're assuming that two injured CMs would be out for the whole season: highly unlikely in the first place - more likely that, worse case scenario, two would be out for only a short time together and that maybe one would be long term....and in any case we would still have Besic to augment the two fully fit CMs for however long the two injured CMs were out (or we could retain Barry for another season and have that insurance, and failing that we would have to do what even elite clubs do in those circumstances: turn to the academy).
It's an area over-subscribed for us in all likelihood. You cant have a massive MF unit: a) it will just cause discontent amongst those not getting game time; b ) we need to be targeting LB and RB replacements/deputies a new CB and a massive marquee signing up front for when Lukaku leaves.