Peter Grant thought Scott Brown was on the verge of being a liability during his coaching spell at Parkhead, now he believes the midfielder is on course to becoming a "Celtic great."
Brown has started the season on top form for Celtic and Scotland after noticeably improving his game last season despite fitness problems.
And former Celtic midfielder Grant, who coached the team under Tony Mowbray, feels Brown is benefiting from a new-found maturity.
Grant said: "Last season and this season he has been outstanding. When Tony made him captain, I thought at the time it was the wrong call because I didn't think he was disciplined enough to even be in the team.
"So what he has done in the last 18 months has been outstanding and he deserves all the plaudits he gets.
"I think he is getting better and better. His game has matured, I think he has matured as a person and that is showing in his all-round performances.
"I have seen him in European games playing exceptionally well and I think that's all down to having a more disciplined approach to his all-round game. He is reaping the rewards and he has been fantastic."
Brown has only been booked once in 12 games this season and Grant admits he is "100 per cent" surprised to see the turnaround in temperament.
Grant said: "He is a massive, massive player for Celtic in and out of possession, his passing is a lot better and his selection of pass is a lot better.
"All these things make him a more rounded player.
"He deserves to wear that armband and if he keeps playing the way he is, he could end up being a Celtic great."