I think it’s too early to say if his decline is physical or mental (or both).
He doesn’t seem to have the same level of confidence in his game and definitely does not run at defenders as much. I hope, because of his age’ he’ll improve in time. I’ve looked up some players who had the same injury (e.g. Beckham, Barnes etc) and none of them were the same players they used to be after their injuries, although they were much older than Callum when they had their injuries.
I’m sure Lamps and his team will be keeping a close eye on him and how best to support him! I’m actually more worried about RLC!