PLEASE search the forums before making a post. There are at least 6 posts of the exact same. Below are a list of a few potential cheapish CM's that will easily do a job for you at united. At the bottom there are a few links in which players are asking for similar things as well as hints of who they have purchased over the time.
Kwando Asmoah
Toni Kroos
Ruben Yttergard Jennsen
Milan Badelj
Kara
Ze Roberto
Jali