United youth product Welbeck joined the Gunners on transfer deadline day following the loan arrival of Monaco striker Radamel Falcao at Old Trafford.
Ferdinand himself ended a lengthy 12-year association with the Red Devils in the off-season when he was released, and subsequently signed for top-flight newcomers QPR.
The veteran defender is of the opinion that Welbeck wasn’t able to flourish at United as he was behind Robin van Persie and Wayne Rooney in the pecking order.
However, the 35-year-old is backing the new Gunners signing to realise his full potential, while suggesting that his former employers may come to regret selling “one of their own”.
“I cannot believe United let Danny go, especially to Arsenal. That seems mad to me,” said Ferdinand.
“He’s a real threat to defenders and, if Arsenal use him right, he will be very dangerous for them.
“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 think he felt frustrated at United because, even if he did well, Robin van Persie and Wayne Rooney would be ahead of him. 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 4x100m team a run for their money.”
Commenting on the Red Devils’ recent transfer dealings, he added: “It is a change of direction for United letting one of their own go.
“Traditionally, this was not their way, adding so many players in a short period of time and having such a radical overhaul. Normally, as with me when I joined in 2002, it was about adding one piece to the jigsaw.
“Some fans still romanticise about their success and the way they brought through so many home-grown players. Unfortunately, you can’t always have that fairytale.
“Radamel Falcao and Angel Di Maria are world-class players and they have added quality in Ander Herrera, Luke Shaw and Daley Blind, but I do wonder if they will live to regret not keeping Danny.”