IMHO you need to consider how many starts you need from your midfield. This is a question that is answered by looking at:
a) how many games the club will be involved in
b) how many midfielders are needed in the manager's preferred system
c) how many midfielders currently at the club can play a midfield role AS DEFINED BY THE SYSTEM.
We seem to currently play better in this 343 / 3421 system, with Ross either up as one of the front three (343) or tucked in behind Lukaku and next to Mirallas (one of the two in the 3421), so for me Ross isn't part of the discussion re the CM spots. If we are going to see any significant use of a 433 then perhaps he will see time in midfield, but only if someone comes in to play in the front 3 with Lukaku, Mirallas and DCL (and Deulofeu, if he stays). We aren't going to stay using the 4231 because we don't have a decent 10, and we are threadbare out in the wide positions.
The candidates who can be relied upon to be fit to start 75% plus of the games at CM in a 343 / 3421 are:
Gana
Schneiderlin (it's not hard to spell, lads)
Davies
and that's it.
But fitness isn't the only factor. Davies needs to be phased in gently, rather than suddenly thrown into the deep end and be expected to make 30 starts across the Prem, the cups and possibly the EL. Alongside this phasing in, we must consider the phasing out of Barry; as it stands, he cannot reasonably be expected to play a full 90 minutes on a regular basis. Over the course of the next year I would hope that Davies will get eased in at the same speed that Barry is eased out. In this time period, Davies should get a few starts and a lot of sub appearances.
Gibson is obviously done. Some of us feel he has been done for a good couple of seasons now, but that's another debate. Get rid.
McCarthy.... better fitness record than Gibson and Besic, but not as good as necessary to be considered reliable to start 30 games a season. Has no impact going forward (which in fairness is a fault you can also lay at Gana's door) but is not as effective when sitting back like a DM as Gana is, or Schneiderlin was for Soton. Schneiderlin was also better going forward, so McCarthy isn't going to be a regular starter once Gana gets back from ACON. Nevertheless, he is likely to command a half-decent fee if put on the market because he's a known quantity in the Prem and he has shown flashes of solid play under Koeman.
Besic has a fitness record almost as bad as Gibson's but has shown flashes of attacking potential, something McCarthy completely lacks. His injury history means you won't get anything worthwhile in terms of a fee for selling him.
For me, the selection is to keep Besic and sell McCarthy. You get a decent fee by doing so and IF Besic gets fit you may have a player who can play as more than just a DM. If Besic doesn't get fit then you sell in the summer and replace him. Selling Besic now raises little cash (more likely to be a loan with option to buy, given he has been out injured for so long).
EDIT: Having taken a look at the fuming on Blue Moon, maybe we offer McCarthy to Man Citeh for £20m? LOL.