It's more a physical thing. He's not lacking in strength but he's always been fairly short and it just seems clear that one more season is exactly how long he'll need to be 'ready' for the first team to the same level that Januzaj was ready last summer.
He's such a sure thing though. I've felt that way since the first time I ever saw him, back when he was 16 or so. When the ability is obviously there, it's the attitude that really decides it for you. Januzaj's attitude was apparent in the way that he so quickly got a handle on each new age group and level of football, and each new challenge he was faced with (such as having to play as our striker in the FIFA Youth Cup, and ending up with the golden boot). For Pearson, his attitude is apparent in the way he takes on each individual game. His sheer force of will is incredible for a youngster... Keane-like, I'd go so far as to say.
EDIT: To be clear, I want us to be giving him and Rothwell League Cup chances and things like that this season. Just not throwing them into being first-team regulars the way we rightly did with Januzaj last season. Because despite the impressive showing in that game where they matched and just outplayed Bentaleb and Dembele, I think they'll need a gentle introduction to the physical robustness of the adult game.
Januzaj, as a winger/AM, basically encounters physical challenges in the form of fouls, so you can throw him in without him being able to 'physically compete' with the big defenders he's up against. But a midfielder - especially a deep midfielder like Pearson - has to be able to match the strength of his opponent, because he's unlikely to get a foul just for a bit of shoulder barging and jostling. It would damage his career to be thrown in too soon and look too lightweight in a number of matches in a row, with opposition midfielders shrugging him off and running clear at the defence.