The Scot's former deputy Rene Meulensteen has revealed that bids for all three players, who were on the books at Borussia Dortmund at the time, could have been tabled if Ferguson had chosen to remain at Old Trafford.
"The fact that he was going to retire or finish came to me and a lot of others as a complete bombshell," Meulensteen told the Manchester Evening News. "I didn't read anything in him where it suggested it would be his last season.
"We had very normal staff meetings regarding pre-season. That year we were going to Australia. Sir Alex even mentioned taking us to one of the best vineyards out there. I was looking forward to that.
"We had possible scouting meetings, possible players we were looking at in terms of trying to build a picture of what we think we need. All that was ongoing.
"We looked at a few German players like Hummels, who went to Bayern Munich. I think we looked at Gundogan and Reus, too. There were a number of players but we never got that specific to really target them."
Hummels left Dortmund for Bayern Munich, Gundogan is now with United's local rivals Manchester City, and Reus is the only member of the trio to remain with Dortmund.