Oh I definitely agree, the Mkhi replacement in of itself is not an urgent need per se, but I do think we need someone to back him up other than Lingard, who would be much better served as the multifunctional attacking substitute a la Blind in defence and midfield. Plus, once we transition away from Rooney, Mkhitaryan could move inside to the 10 position, and then we would have no one to play on the right to an equally high standard.
Either way, if we move Mkhi inside or not, we need another body in there and for me, it should be a youngster who would be happy to spell him in a squad role for a year to two before eventually replacing him. Someone with the right profile is essential here: young, fast, left footed (for sake of balance), and carries a real dribbling threat to mirror the threat of Martial on the opposite wing, and would also allow our attack to continue to interchange their positions during periods of dominance, as we saw on Sunday briefly.
Also, the Giggs analogy; Giggs is probably the best pure dribbler of his generation (Messi is in the succeeding generation). No other winger has played at that long at that level. If Mkhitaryan is serviceable in a wing role for us by the age of 32, I will be glad. Amazed, but definitely glad.