As somebody else commented, we were caught yesterday with not good options from the bench. Having lost Gana, we had no defensive midfield cover for Sanchez and Westwood who would tire as the game progressed. Sherwood should have had a plan for this eventuality as it would have been a high probability. Rightly in my opinion, somebody commented that Veretout being thrown on in that position would have been the equivalent of throwing Ayew into the game. We are still short in central midfield when we have injuries, so there has to be another plan.
How is that then? If you're happy to accept that we were missing additional presence in central midfield, why would putting a highly-rated, energetic central midfielder on be the equivalent of putting a wide-forward on?
Clark (as you mention) and Richardson have both played that position for us in the past as well. Ayew didn't come on because there was no central midfield options on the bench, he came on because Sherwood didn't think that the central midfield needed strengthening.
And a minor point - we didn't necessarily need to replace either Sanchez or Westwood, we needed to give them additional help. Veretout running around or even Richardson just chasing Mahrez would have made things much easier for them. See West Ham last night. 2-0 up after an hour, take off Lanzini and stick another defensive midfielder on. See the game out at 2-0.
The comment was meant as a new player not yet up to speed as opposed to positional (midfield compared with forward).
Again, I didn't say replace Sanchez or Westwood, I said cover them as they tired i.e. strengthen the position.
I think where we struggle with the squad is that most of our players are one position players or if they can play other positions, they are quite a bit weaker. This means that we tend to replace like for like.