Mangala is not the answer. Not right now. He played well against Chelsea and needs to develop his game but he still appears somewhat of a liability to me. I think Otamendi would be a great signing. Gotta think about the Champions League. Kompany and Otamendi would form a solid partnership with zabelata and kolarov either side.
Otherwise, the only other position to strengthen is the right wing. I have never seen a winger hit the first man ever! He has blistering place but always checks his runs, tries to cut back inside, has no left foot and just ends up stopping and stalling. I thought he'd be a great success when he first joined but I am sorry, Nasri is a far better player. Pedro would be an ideal signing and De Bruyne would be a quality. With Delph and Fernando to return, defence and defensive midfield looks rock solid, with serious quality up front. However, a lot still rests on Aguero and staying fit. Bony is class but another striker is required.