It would make no sense to let him go in January. He would be cup tied anyway nothing will happen before the summer. Should we manage to win something whose to say he won't have a change of heart?
It's not worth entertaining that thought. The fact is that Barca are too far ahead of us as a team at present for us to rely on a South American to contemplate wanting to stay put. Compare with City, who kept Aguero and David Silva from Barca and Real precisely because they were winning things. We need to win a few things.
What we do have going for us, though, is that
1) There's no way that FSG are letting Coutinho go in January, unless it's for £250m.
2) Barca want Griezmann and have a lot of financial trouble. They will struggle to pay for both Griezmann and our likely asking price of £150m (probably rising to £200m by the time the summer comes, and Coutinho has established himself as one of the top 5-10 players in the world by then) in a single summer. Especially if they want to make a third or fourth signing, as they probably need to do, in order to get their squad depth to a decent level.
So I think there's a good chance we have him for another season after this one. After that season, there's no telling what could happen: Barca could sign someone else for his position, a new president/coach could come in(?), we could win some major trophies, etc.