Former United No.2 Steve McClaren has said whoever takes over from Carlos Queiroz must be of the highest quality.
"The new assistant will have to above all have experience and be a very capable coach," McClaren told BBC Sport.
"Working with the players could be quite daunting, but I found it very easy because of their quality and the way they went about the job.
"They just wanted to improve, and as long as you can improve them, they will accept you.
"I developed a tremendous amount under Sir Alex Ferguson, you can't not.
"But it isn't just Sir Alex, it's the players as well. If you can't learn from that sort of experience, then you can't learn from anything.
"The one thing Sir Alex always did was employ ambitious people. Whether that is the coaching staff or players they always had to have ambition,"