TBH (to be honest), I'm surprised if Carey has agreed to sign on the basis that he'll be competing for a place, as he was looking to leave Saints in January on the grounds that he didn't think he was getting enough 1st team action. That said, with only a few days till the season KOs, he'd be looking to get fixed up.
For me, he is the type of player who needs a consistent run (in a consistent position) in a team. He can be a bit erratic, but is undoubtedly a very talented player.