Look, if we sign no one else, are we really that bad? We have a very solid core in Pogba, Sanchez, Bailly, Lukaku, De Gea, Fred, Matic, and some great players in Rashford, Martial, Lingard, Lindelof, etc. We severely under-performed last season and I for one, am willing to believe our players have a lot more to give and they will.
EDIT: In regards to our fullbacks, Valencia and Young barely put a foot wrong last season and ironically are two of the fittest players in the team. Arsenal signed Lichtensteiner, who is 34. Age is nothing but a number.