Dont have a good backup striker you mean ? surely you're not saying Rooney isnt a good striker. Not much between him and Aguero and imo. The 2 best strikers in the league; play them both in their positions for a full season and you have 20+ league goals in the bag from them. They do need a backup and I wouldnt rule them going after a big name out just yet. They let Persie go for a reason and my guess is that they already have someone lined up.
They have decent CBs but should still be looking to improve there. Losing De Gea would be a big blow though.
When it comes to midfield, I think their latest signings give them the best midfield in the world in club football.
CMs: Carrick, Schweinsteiger (Blind, Schneiderlin, Herrera)
AMs: Di Maria, Mata, Depay (Young, Januzaj, Valencia, Fellaini)
If they can add Ramos and a backup CF, they'll have the best squad in the league imo.
I'm more worried about Arsenal to be honest who -realistically- are the most likely to finish 4th next season. They have added Cech and I think with one or two additions they drift further away from us and closer to the likes of Utd, City & Chelsea.