Former Manchester United midfielder Ray Wilkins is concerned the sole addition of Marouane Fellaini in the summer transfer window is not enough for the club to sustain a domestic and European challenge this season.
The Belgian midfielder joined on deadline day in the summer transfer window from Everton and was the only significant purchase made by manager David Moyes ahead of his first full season at Old Trafford.
And Wilkins, who spent five years at United from 1979-1884, believes the lack of additions will hinder both the club's Premier League title defence and their progress in the Champions League.
"I think David will do an excellent job," Wilkins maintained. "But I fear for them this season. I think Fellaini was a good buy I must admit but I think they needed more.
"I think they needed more players particularly if they are going to go on the European front.
"The Champions League, if you look at it every team it has reinforced. United have reinforced to a degree but not as well as I would like to have seen them do so."
A summer window dominated by the future of Wayne Rooney saw United miss out on a host of high-profile additions including Cesc Fabregas, Ander Herrera and Fabio Coentrao, leading to questions surrounding the work of Moyes and new executive vice-chairman Ed Woodward.
United's uncertain start to the season, coupled with a substantial activity of their rivals, suggests this term may represent one of transition ahead of a renewed tilt at the title next term.
Chelsea and Manchester City, joint-favourites for the title in pre-season, have made a host of new additions while Arsenal, Tottenham Hotspur and Liverpool have all significantly strengthened.
Much scepticism has met the addition of Fellaini but former United defender Henning Berg says the ex-Evertonian can replicate the stature of Eric Cantona.
"He's not the kind of player to thread passes through but he has a sheer presence to hold the ball up, ability to win balls in the box and match-winning mentality - he raises his game for the big matches," Berg said.
"Every game for United is a big one, so I think he will rise to that and enjoy it.
"When Cantona came, he had that mentality every game. I'm not saying he's going to be the same, but he can be."