He was never a good defender, no.
He was very good in a man-marking role - as was pointed out- when asked to take on that responsibility. In general play, not great defensively. In his early days, he had a touch of dynamism about himself going forward, too, but technically he was always second rate. He had and has a very good attitude for a player of his status, but he has basically taken his talent to its limit which is to his credit. Right now, and for half a decade now, that level should not and has not been good enough for an ambitious Premier League team.
Coleman was an excellent player, and an excellent all round full back for a good number of years. His attacking threat started to diminish before his defensive skills started to diminish.
He's been an excellent player for the club but he's just not that good anymore. A good player to have around the squad still, but I don't think he can cope with playing every week and it will cost us points.