Quality Double Signing can Seriously Help Manchester United

Manchester United have started their CL group stage with a defeat against Dutch side PSV Eindhoven. The Red Devils are in a tricky group and qualification to the knock out stage is by no means guaranteed.
Not qualifying for the next round will be considered as a massive failure and hence United need to step up big time.
Legendary Red Devil star Roy Keane said (Independent) that Louis van Gaal needs to sign quality defenders if they are to seriously challenge in the Champions League against the best teams.
Keane told ITV:

“Defensively I still think United are all over the place”
“The money United have invested, going forward they will be fine and in midfield they are not too bad but I think they are crying out for centre-halves, definitely one, possibly two.”

The Red Devils were linked (Independent) with central defenders like Sergio Ramos and Nicolas Otamendi in the summer transfer window. In the end, United could only lure one defender in the form of full back Darmian from Torino.
Smalling and Blind have done well so far in the central defense but the Red Devils do not have a world class center back in their squad who can lead the defensive line.
United must splash the cash in the winter transfer window and sign at least one top quality center half to reinforce their back line.