Ferguson retired from his position at the end of last season, with David Moyes taking the reigns at Old Trafford, while chief executive David Gill also stood down from his post.
Moyes and executive vice-chairman Ed Woodward endured a disastrous transfer window, publicly missing out on key midfield targets Cesc Fabregas, Thiago Alcantara and Ander Herrera.
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"If Sir Alex and David Gill had still been the bosses, I won't name the player, but there would have been a star signing, one of the absolute best players in the world," Phelan told a Scandinavian United supporters’ club magazine.
"It was something the boss talked about, and the owners and David Gill were also quite clear on doing it.
"There's very few of such players out there. I think it's clear to everybody who I'm talking about.
"We didn't see the need for a big clear-out of the squad, that's why Sir Alex and I had the one big signing in sight. One such star signing gives everybody a lift, we saw that with Robin van Persie."
Cristiano Ronaldo has been heavily linked with a return to United over the past year,
The 51-year-old’s comments came after the release of Ferguson’s autobiography, in which it was revealed that Wayne Rooney pleaded with the Scot to sign Mesut Ozil while he was still playing for Werder Bremen.
"Ozil was not on our radar. I said that to Wayne at the time," Ferguson said in his autobiography.
"We signed Chicharito [Javier Hernandez] that summer. My thoughts at the time were a Rooney and Chicharito partnership. That was a fair prospect for us to look at.
"When Ozil was at Werder Bremen he played wide right initially then they played him off the front.
"That type of player was not necessarily on our radar. I told Wayne, 'Leave the signing of players to me'. We've got lots of them right."