Rooney is an amazing scorer who has had to carry a playmaking burden. Bringing Sneijder in gives them a better playmaker AND it allows Rooney to be a better goalscorer. I could see them largely playing with Sneijder at 10, Rooney at 9, and 2 of Valencia, Nani, Hernandez, and Young flanking them. Will be interesting to see what they do.
If they brought in Sneijder as a central midfielder, they would definitely have to bring in a De Jong-type and ask more defensive work of their wingers. I'd see him as a 10