The France midfielder is still struggling to overcome serious injury.
"I'm frustrated and disappointed for him with all his injuries. If he could play regular matches, he would have grown steadily and to an exceptional level. Potentially, he is even better than me," declared Vieira.
"Better technically, better dribbler, scores more goals, more comprehensive than me. Maybe I was better in recovering the ball in tackling.
"It must be hard for him not to reach his maximum. But it may not be too late."