Funny you should mention that. United have a worse record of bringing English youngsters through than Man City in the Premier League era. On top of that, Januzaj was the first youth player they'd brought through in general since Welbeck (who not surprisingly was loaned out a few times) as I recall and that was what, 5 years? United's squad was not good last season -- they finished 8th for a reason. When you are a team who is in that area of the table, it makes sense to give some minutes to a younger player who can't be any worse.
Despite having a strikeforce of Rooney, Berbatov, and Chicharito in 2011, United did give Macheda 12 matches that season. He got 10 matches the season before. He had a chance. He just didn't perform.
Januzaj was going to play regardless of where United finished last season. He was impressing too much in the youth teams. I said as much before last year's pre season. Fergie also decided to integrate him with the first team towards the end of his final season which is why he was around during the title celebrations and ultimately became an unused sub in his final match. That was when United were champions, not 7th.
In general, Fergie did get more conservative with youth in his later years. It's one of the reasons why we lost Pogba. What's strange is that Pogba was never going to be loaned out and Fergie said so at the beginning of the season because he was foreign and too much of a talent. No idea why he went back on his word and didn't play him much.
The one thing Fergie constantly did was give youth players minutes in the cup matches regardless of opposition. Against Chelsea United played a cb partnership of Wootton and Michael Keane. His subs were Tunnicliffe, Macheda, and Powell. Lingard, Vermijl, and Brady were all on the bench. United ended up losing that match.
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Finally, people need to stop using championships won at youth level as evidence of the team being really talented. It's rare for a team to be insanely talented and also win a trophy proving that. It doesn't matter where a youth team finishes so long as there are 1-2 great players. A team with a better average of talent will beat a team with 2-3 potential stars + below average teammates yet everyone would prefer the latter team.
Look at Southampton. I don't see them winning trophies at youth level and yet they are producing great talents.