Sure. But he's not lost a season of development. He got multiple first team starts at the beginning of the season. He got more first team starts when he got back from injury. He started again on Tuesday. He's had a season of huge development. Sure it could have been MUCH better but given last season he was on loan in the championship this has still been a really encouraging year.
Certainly, and I'm the first to remind people that there's more to a footballer's job than the 90-minute games we see on the TV. Most of their 'day job', so to speak, is to come into training and work with coaches and medics and developmental staff to improve both their own and their team mates' games; that's where the bulk of development and improvement takes place, not the public-facing window of the scheduled games, and so he's continued to do that.
I was merely addressing the poster's point that just because he was 'back' by Feb or whenever it was, doesn't mean the injury hasn't had a knock-on effect beyond that date. We'll never know whether or not he might have grown into a first choice or first rotation player for this crucial period if he had not become injured. Given his age possibly not. But the injury pretty much knocked any chance of it off the table.
Fortunately he is so young and talented that he has plenty of time to become everything he can possibly be