The former Middlesbrough player came on in the 70th minute of the 1-1 draw at Burnley, his first match since breaking a bone in his foot last May.
Martin O'Neill: "Just to get on the field of play is a very positive sign for him.
"It might still be a bit too early as, in terms of proper fitness, it will be hard for him at the moment because of the amount of football he has not played.
"I'll be the first to tell you he is not entirely fit but he has been desperate to be involved with us.
"He will benefit from that and he can only improve from there.
"He's a very decent player, he's played a number of times for England and he can play.
"But I think if he stays clear of injury we'll see the best of him from Christmas onwards."