The £73million signing from Dortmund has struggled since arriving last summer, is currently working under his third manager and his failure to make England's World Cup squad was not even a talking point during Thursday's announcement.
Parker fears Sancho, who racked up 45 goals and 53 assists in 116 appearances for Dortmund, will never fully adapt to Premier League football having turned into a player afraid to take on defenders.
"To be honest I’ve never really understood why Man United paid so much money for Jadon Sancho," Parker told Bettors. "He has a lot of issues because he is never attacking full-backs, he is passing backwards and is not really challenging his direct opponent.
"The way you play in Germany is completely different to the Premier League and it seems like he’ll never be able to adapt well enough to be a key player for Man United.
"If you look at the money Man United paid for him, I would say that he could end up being the worst buy ever in Manchester United’s history."