Up until last year I believe, the NFL ruled that a WR had to be assigned an 80s number unless they received special permission by the NFL (Keyshawn got this).
However...
Last year the NFL ruled that WRs are now allowed to wear numbers in the teens (10-19) as well as the 80s, hence the reason last year a bunch of the rookie WRs decided on the teen numbers (guys like Roy Williams, Larry Fitzgerald, Reggie Williams).
So Roscoe can stay with #11 if he wants, the NFL wont force him to give it up.