Because he's 19 and should not be playing an excess of matches. I made the point before that man child, freakishly developed teenagers get away with it and come out the other side untarnished. Players like Milner, Barry, perhaps the truncated career of Giggs get away with it, but more commonly, by some distance, teenage sensations are ruined by 30-odd and limp out the other side of their career a shadow of what they were. Owen, Rooney, Kluivert, Fabregas, Torres, Wilshire, Joe Cole, Whiteside, Richards, the list goes on. Rooney wasn't even handled poorly, and one might argue that his lifestyle played the bigger hand, but the point stands; just because they burst out the traps, it does not mean they should be played as if they were fully developed men.
27 full matches worth for a 19-year old is a lot of games, particularly for a forward at a top club, which should not be lost in this discussion.
The minutes they do get should be tailored to their mental and physiological makeup. Granted, I don't know what Greenwood's constitution is, but he doesn't look like some genetic freak to me and I don't think his playing time in this team at this time, with Ole's predicament are at all predicated on that. There is absolutely nothing wrong with coddling and nurturing the talent and slowly bringing it to the boil, which is normal trajectory for far more of the great ones than not, many of whom were non-entities as teenagers.
Initially is quite nebulous as a terminology; what do you mean? Over how many games do you think a manager desperate for points would stick with underperforming players when he has options to flip that on its head and try someone different? How many games do you think a below par Rashford would get with the pressure cranking up on Ole before he's led to the guillotine?
Even if you take Rashford out of that and suppose it's Martial tanking, how long do you think the manager would persist with him before giving a player who is a proven goalscorer a go?
I cannot see why this is black and white to you when there are so many variables that would have ensured Greenwood playing time. The OP is even questionable if Sancho came in and was on fire - there are too many games in too little time for us not to rotate between the starters and what we have on the bench. In a normal, non-condensed season, all of them at peak form does lessen Greenwood's playing time, but even there, 10-15 starts are not out of the question. You factor in Rashford's back, and that number very probably goes up.I don;t think any of this is a leap or a stretch, rather, elementary conclusions to make - ones brought about via necessity and not trying to stuff Greenwood into the team as an afterthought.