He didn't develop anyone, but he gave a lot of them chances, which is great, but I doubt that was his plan. In a perfect LVG world the 1st choice players would always play, usually regardless of form.
Basically his strategy is to play the first team as much as possible, to do that you can't really have a large squad, because back-ups don't get much playing time and they get frustrated. So the only logical option is to keep a small squad and call up some young players when senior players get injured, because young players are usually more patient and they are grateful for the time they play instead of being dissapointed of the times they don't, like the more established players.
What I think separates him from most of the managers and what I like about him is that he usually prefers a specialist instead of playing an experienced player out of position, so I guess this helps the youngsters get more games.
Unusual strategy that!
Injuries could always predicted. Long-term injuries are always a possibility. What we are seeing is his plan in the event of such occurrence. Unless we are implying that he could never have predicted these injuries, which is extremely arrogant and an insult to a manager of as much experience as him.
As for the debate of him 'developing' them - I think the work in turning 17 and 18 year olds in pros who can translate academy promise into first-team form is part of development. Not their early development, of course the likes of Joyce does great work too - but they are still in development stages when they come over to LVG. Of course, he wasn't their under 8s coach, but the likes of Joyce didn't coach them for many years either. They change coaches as they move up a level every couple of years, and Van Gaal is just the next one on their journey, and the most important one.