Manchester United Supporters' Trust (MUST) head Duncan Drasdo expects "disquiet" among the club's support after Alex Ferguson's retirement.
The Scot revealed on Wednesday that he will bring his 27-year reign at Old Trafford to an end when the Premier League season reaches its conclusion on May 19.
And MUST chief executive Drasdo believes fans will harbour fear over life after the 71-year-old, who has lifted 38 trophies during his time at United.
"(Fans are) distraught, (there's) utter despair," he said. "People are immensely grateful to Sir Alex for what he has contributed and so first of all fans want to say thank you.
"But obviously there's going to be a sense of disquiet and fear about what the future holds, who's going to come in, are they going to be able to maintain anything like the success Sir Alex produced in the 27 years he was at Manchester United."
Drasdo went on to claim that the news had been partly expected, especially considering Ferguson's dismay following his side's elimination from the UEFA Champions League this season.
"It wasn't a total surprise," he added. "Obviously we heard last night that it was very likely that this was going to be the big announcement. We'd also seen some signs that this was going to be Alex's last season.
"I know it's easy to say these things in retrospect but I certainly felt at the time when we saw him walking off the pitch after the Real Madrid game that he was totally distraught and you just felt that maybe that was the feeling that it was his last chance to win another European title.
"The other factor was that we'd been told by a pretty good source that when David Gill went that Sir Alex would go at the same time."
Drasdo, whose organisation states "fan-ownership" as one of its main aims, also discussed the differing qualities of the two men most strongly linked with replacing Ferguson.
"I think it's a straight toss up between (David) Moyes and (Jose) Mourinho and certainly you can see a logic to either of them," he continued.
"Mourinho is obviously a supremely confident manager, confident in his own ability – some would feel too much at times.
"He could walk into any football club in the world and feel he could take on that job no matter who had been in there before him.
"Then there's the other model which would be someone like Moyes, who you might say is cut from the same cloth as Sir Alex.
"You would see that they would have a close relationship where Moyes would feel comfortable talking to Sir Alex, learning from him, particularly when you're talking about moving onto the European stage where does haven't experience. You could make arguments for both of them."