I don't know what Cancelo is, actually. He's a left back/right back/false LB/RB/midfielder - who's awesome for us.
That said, if we signed a traditional LB next window I think that it would improve our team substantially. Mendy is hopeless, Zinchenko is OK but I don't rate him as a top choice candidate for starter at LB in any top club (he's good backup).
So for LB/RB we currently have:
Very good, top choice:
Ake (I'm pretty sure, though I've not seen enough of his play)
Laporte (I think - though I'd rather have him playing CB)
So, IMO, we lack top-quality depth at FB and if we can sign a top quality LB next window it will improve us.
First priority next window though must be a world-class striker to replace Aguero.