Maybe because Januzaj has been playing well in training? He is more of a natural wide player than mata or herrera.
Just because he hasn't played great against city or Chelsea doesn't mean he doesn't deserve to start against relegation battler.
These players sometimes need to play themselves into form. I don't think he deserves any less game time than Mata (or for that matter - most of the squad).
I think it's good that he is is playing and I hope he has a good game.
I don't think herrera is 100% yet either, LVG probably doesn't want him on which could leave him more vulnerable to more time to put.