Barcelona should keep Depay or find a replacement for him if they are eager to part their ways with him in January already. Because Lewandowski won't have a striker as a replacement for himself if he Depay leaves. I know that there are players like Ferran Torres who can play there but it wouldn't be the same effect certainly.
This is important for them not only in the La Liga but also here. The Europa League title must be among their targets in the end. They need to keep the team strong for that.