Carrick has another 2 years in him. But yes, we'l need to look for a replacement for him too by the end of next season in all probability. Hopefully it will be a young player that we sign that eventually takes over his role rather than needing to sign top CMs every summer as they are pricey and tough to get. As it is, without CL next year, we cant hope for too many top players without paying obscene wages. A Song and a Kroos/Gundogan level will be fantastic summer.
On what evidence? he's not been very good this season. Although, that could be just down to form, rather than a sign that he's declining.