Manchester United legend Rio Ferdinand believes Danny Welbeck's frustration was the reason behind his switch to Arsenal.
The England international left Old Trafford and completed his switch to the north London club on a permanent transfer after he was informed his services are no longer required by United manager Louis van Gaal.
Ferdinand was also asked to leave after his contract expired at the end of the last season. The central defender joined Harry Redknapp's Queens Park Rangers on a free transfer and will return to Old Trafford on 14 September, this time as an opposition player.
The QPR defender revealed his surprise after the former Barcelona and Bayern Munich manager allowed Welbeck leave United.
"I think he felt frustrated at United because, even if he did well, Robin van Persie and Wayne Rooney would be ahead of him," Ferdinand told the Sun.
"Danny has everything to be a top player. English football has yet to see what he can really do because he hasn't been getting a run of games."
"I cannot believe United let Danny go, especially to Arsenal. That seems mad to me," he added.
Welbeck now joins his compatriots Theo Walcott and Alex Oxlade-Chamberlain and summer signing Alexis Sanchez at the Emirates. Ferdinand believes the Gunners' squad will have a lot apace and has backed his former teammate to shine at Arsenal.
"He's a real threat to defenders and, if Arsenal use him right, he will be very dangerous for them," the QPR centre-back stressed.
"At Arsenal, he will be the main man and I have no doubt he will flourish. Imagine the pace Arsenal will have when everyone is fit. There will be Alex Oxlade-Chamberlain with three fliers in front of him in Theo Walcott, Alexis Sanchez and Danny."
"I'm telling you they would give Usain Bolt's Jamaican four-by-100-metres team a run for their money," Ferdinand concluded.