The French midfielder was signed a year ago from Lille for a fee of around £4m after escaping the attentions of Europe's top clubs.
Carr told the Sunday Sun: "When I look back at my time with Newcastle, I would have to say that, pound for pound, Yohan has been the best player I have scouted.
"He was one of those guys you see and right away make up your mind about his quality.
"When you consider how good he has been, and what the club ended up paying, then you have to say he has been fantastic.
"Although Newcastle have been good for Yohan because he may not have been at the Euros at all if he had stayed at Lille."