Not perfect, but in this moment I would rather keep him than let him go and buy no one.
Coutinho is not perfect, but can offer some things and we are on a verge of reaching a CL final with him as a starter, so he can't be that bad.
Dembele, also has a lot of good sides and lots of huge flaws.
Each of them can offer some things under different circumstances.
Imagine if we sell him and play a CL semis in the next season.
With Dembele, you never know. If he'll be in his form=fine.
If he will be injured, raw after injury or his disconnected and poor form self, he is almost unplayable.
Then the only option is Malcom, who is still quite questionable for the highest level.
So, if we don't buy a new winger, I would keep Cou.
I just can't rely ONLY on Dembele, on his injuries, slow time to getting back into rhythm and his decisions making.