Former Everton striker Graeme Sharp insists time must be given to David Moyes, who has been put under pressure since he replaced Sir Alex Ferguson in the Old Trafford hot seat.
United have lost three of their opening six matches in the league, including a defeat to arch rivals Liverpool.
The Scottish manager has received criticism for his dealings in the summer transfer window, with reports of failed bids for Gareth Bale, Cesc Fabregas and Spanish star Ander Herrera.
Marouane Fellaini was brought to Old Trafford on deadline day for £27.5million from Moyes' former club.
And Sharp, who became close to the Scottish manager during his time at Everton, claims the 50-year-old will need to spend during the January window.
Sharp, speaking to talkSPORT said: “When you look at David Moyes’ squad, he needs to strengthen, he was after Cesc Fabregas and Leighton Baines, among others, but it just didn’t happen.
“I think they’ll be really active in the January transfer window if the players become available.”
United travel to Ukraine to face Shakhter Donetsk in the Champions League on Wednesday having previously beaten Bayer Leverkusen 4-2 at home in the competition.
Despite the concern from fans over the Red Devils' form, the former Scotland international added that the United faithful would have to be patient.
“United fans have to be patient,” he said.
“It doesn’t matter who was in there, it was always going to be difficult. Everybody has said, ‘Oh well, they won the league last year’ but it’s not the best of Manchester United teams.
“The other teams have strengthened well. It’s going to be difficult for David Moyes to retain the championship this year.
"They have got to give him time to mould his own side now. It was always going to be difficult for him replacing Sir Alex Ferguson.”
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