It was very well documented that Joao Felix had a tough start to life in Spain. Also Mbappe's had plenty of struggles and obstacles to overcome, not that anyone on RedCafe has ever taken any notice. Also it remains to be seen how Haaland would deal with a dip in form, he's not had one yet but it's inevitable.
Have a look at some of the Golden Boy recipients over the last couple of years and you'll notice a trend with them that applies to Greenwood too, Mattias de Ligt had a shaky spell after moving to Juventus and made some very high profile mistakes and has since recovered his form and is one of their best defenders, Mbappe and Felix have already been discussed, Raheem Sterling had a barren adjustment period towards the end of his time with Liverpool and his first season at City was a disaster and is now regarded as one of the best wingers in the country, Martial went through a similar patch to Greenwood (that actually spanned mutliple seasons) and finished as our top goalscorer last season and as
@acnumber9 mentioned you've had endless excuses for him.
Looking at some of your other examples, Cristiano Ronaldo at 19 was incredibly streaky, us United fans should know that better than anyone else. As was Wayne Rooney who went on to be one of our best ever players.
Young players go through these poor patches of form, of course it's a valid excuse for his form seeing as almost every high-profile teenager has spells in and out of form, it's how they overcome that form that seperates players like Mbappe from Renato Sanches.