Answering your question directly, probably a guy who does his best work deeper. That's pretty much what I've been basing my comments on anyway really, the balance of the squad.
I'm presuming Whelan and McCarthy are going to start the games so I do think we need cover for those, whether it's to bring in fresh legs, cover for potential injuries or even replace one of them completely if they're really struggling. I think Gibson or Meyler would add a bit of insurance here. All of the other midfielders that we'd consider 'unofficially confirmed' are of a more attacking nature, as well as having the 'switching Brady' option available.
Obviously there could be a situation where it would be better to have Arter in reserve, if we're really chasing a game, but I think we have more to work with in that situation than we do if McCarthy pulled his hamstring in the first game.
There's also the reliability factor, I have no doubts that Meyler (or Gibson) would have the temperament to just slot in. Arter might too, but he just looked like a guy that was trying too hard against England. It will be interesting to see how he performs tomorrow, hopefully he starts as it's tougher coming off the bench and feeling you need to impress straight away. If he's as good as he looks at times for Bournemouth then it might change my mindset some bit.