O'Neill told avfc.co.uk: "He's a strong centre-half. He played a big part in Rangers' run to the UEFA Cup Final last year. He played exceptionally well in that.
"He hasn't got any particular frills about his game but I think that he is strong, he wants to compete and, as importantly, he wants to go and see if he can make it in the Premier League.
"He's got this driving ambition to play for Spain, which is no bad thing. It would be nice if he was able to do that and he feels that perhaps if he can play regularly in the Premier League he'll have a better chance.
"He's got plenty to prove, but haven't we all. Luke Young, who's played for England and Nicky Shorey who's played for England have both got things to prove. I think that's pretty important and there's a good determination about those players."