The Scot insists the club still boast the financial resources to attract the world's top talent, but admits his key targets chose other clubs over moves to Old Trafford
It emerged in September that the Red Devils had made a £125 million bid for Gareth Bale during the summer, only for the move to collapse because the Welshman already had his heart set on a move to Real Madrid.
Public pursuits of Thiago Alcantara, Cesc Fabregas and Ander Herrera also failed, while reports frequently linked Cristiano Ronaldo with a United return until he signed a new contract at Santiago Bernabeu.
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"We were very close to a major, major signing," he told reporters, without revealing the player's identity.
"It's easy to say now, 'Well, you didn't do it' but we were very close to a major signing when the club would have definitely shown what its worth is.
"We have got the money and the club will spend. There is no question about that.
"We were close to a couple of really major signings [in the summer] but, in the end, we didn't quite get there... Players have chosen other clubs or made their decisions elsewhere."
When asked if he would make another move for the same player in January, Moyes replied: "Not that particular player. I would be very, very surprised if it was [to happen] next summer, either.
"I want players going forward and not just for the next six months. If I can get someone to be with us for the next six or seven years I want to bring them in as soon as I can.
"But a lot of [potential targets] may well be at clubs in the Champions League or at clubs who are not willing to sell because they may be in a good league position at this moment in time."