Edwin van der Sar believes successor David de Gea is one of the key reasons behind Manchester United winning last season's Premier League.
Van der Sar closed out his career in style at Old Trafford, winning four league titles, two FA Cups and a Champions League before hanging up his gloves.
De Gea was given the unenviable task of the replacing the former Holland international two years ago, with United making him the world's second most expensive goalkeeper aged just 20.
His performances after joining from Atletico Madrid were heavily criticised, but he has risen in stature and shone as Sir Alex Ferguson capped his final season with the league title.
De Gea's performances saw him selected for the PFA Premier League Team of the Year and Van der Sar is full of praise for the man that replaced him.
"It is unbelievable," the ex-goalkeeper, now Ajax marketing director, said. "He came from different league, he was young. He was 20 when he came.
"He came from Spain, he didn't speak the language. There is different food, different tactics, different methods of training.
"Corners and free-kicks, balls in the box - that is different to the Spanish league. It was normal he could have some troubles in the beginning.
"I think the way he turned the corner from January onwards has been a real benefit for United and one of the two or three reasons they won the league.
"His physique is better. He worked really well with (former goalkeeping coach) Eric Steele and the physical coaches and that is the way you improve.