Castro was good enough to get a contract at WBA so going on loan to a NL side was for fitness and game time so of course he would expect to get into the starting 11 and that not being the case why wouldn't he go somewhere else where he plays regular football - I certainly don't think he had bad attitude.
Totally different player to Bostock he hasn't discovered the defensive side of football yet.. perhaps JB hadn't at his age, showed glimpses of skills above NL and good attacking intent but didn't work hard enough for me - see Ruben's workrate when we don't have the ball.
JB is a key player for us next season and wouldn't be surprised to see him as a double pivot with MP especially in away games and tbh don't think we could get anyone better in that position at that level.
I wouldn't be surprised if we don't receive offers from Division 1 or Championship this summer for him - for me a fit JB could play Championship probably more than any other player in our squad.