The Scotland left-back, signed from was exceptional after the Premier League restart.
"He was outstanding at Celtic for many a year, played lot of European games,' Hartson told talkSPORT.
"Champions League games, picked up trebles and trophies. He's clearly a winner.
"He's an outstanding player, he gets forward, he's got an FA Cup winner's medal under his belt, a Community Shield medal.
"Initially he was injured and was in and out a little bit with his groin, he couldn't quite find his best form.
"But now that he's settled he's a future Arsenal captain for me, 100 per cent."