For the next 6 months we are fully in control of Alexis' destiny. City were prepared to pay 20m for him according to reports (that represents a 'value'). Alternatively, we could have decided to hold him to his contract and forced him to stay, we still want to win the league cup and Europa league after all.
I personally think we should have sold him and brought in a replacement early in the summer or made it clear to him he should respect his contract and leave for free at the end of the season just like Dortmund did with Lewandowski. We did neither, insisted early on he wasn't for sale only to then try to sell him to City on the last day. In the end, we were left with a very disappointed player who had already closed the Arsenal chapter in his mind. Very poorly handled by the club.That's not a shock though.
By the way, United may well have tried to sell Mhikitaryan in the summer but we know from experience it's not that easy to sell underperforming players on high wages.