Rashford looked good when he first broke through and has since stalled as a player. There are other examples, agreed, but there are also examples where it amounted to nothing such as Januzaj, Wilson and Macheda all looking very good initially then fizzling out.
I like Greenwood, I think he's the best we have in our academy at the moment (well, a toss up between him and Chong) but I don't think we should be putting him in the first team, he hasn't filled out yet so the strength he employs well in the youth team would be wasted and it's a very important aspect of his game, he's sixteen there's no rush, maybe 5 minutes here or there wouldn't hurt but that isn't what you said initially.
And people are only tolerant if they start well (see Januzaj/Rashford), Lingard and McTominay both have had very audible critics since day one. It took some exceptional form from Lingard last season to temporarily silence them, Rashford also has started to grow a fairly sizeable amount of critics and he's someone who did start well.