The Cameroonian international is coming under increased pressure at Trabzonspor after a disastrous performance against Fenerbahce this month, which has put his permanent move to the Turkish side in jeoprady.
Onana failed at United and was often cited as one of the worst player to ever pull on the shirt for the side due to his countless errors and goals conceded whilst at the club.
The 29-year-old, whose struggles continue in Turkey, has been defended by legendary goalkeeper Van der Sar this week however as he blamed the state of he club for his failings.
“I thought, and I still think, that he has tremendous qualities – reflections and his feet – but somehow, he has the odd mistake in him that makes you think 'That’s crazy, how can he do that'.
"A team wants stability, to know what your goalkeeper is doing so the back four can relate and I think that’s what happened (the issue) with André.
“United haven’t had stability for the last six or seven years – the back four changing, the centre-halves and midfielders changing, coaches changing – so, it’s difficult for new players to come into an environment where the expectations are high, not only for goalkeepers but also for a winger or midfielder.
“A lot of players who have come here in the last eight or nine years have not reached the level that people expected them to reach.”
Onana has made 21 appearances for Trabzonspor so far this season, keeping just four clean sheets as it becomes harder and harder to defend the former Ajax star.