Sentiment is a wonderful thing. I had Ruben Loftus-Cheek at Chelsea who had beastly attributes but could never complete a single game. Despite that I kept him in the first squad and always put him on the bench as I like him IRL. With Januzaj it all depends on if you're sentimental about him.
If you're not, then cash in as he is of no use to you and you have replacements. Does his scout report say he is injury prone? If not, then he may just have an unlucky spell with injuries and could get over it. Is his training workload heavy?