He's not just praising a player. He's literally gone out and pointed to Greenwood as a player who will contribute to the goals we will miss by shipping Lukaku. Sure martial and Rashford too but the change this season is Lukaku out and Greenwood in. Thats more than just praise. That's banking on a 17 year old, which can be well justified or appear moronic (to use your own word).
Now when he name drops Greenwood (again) in the pre match presser as the man to step up and with Martial out, and then goes on not to start him, reducing him to a 13 minute cameo, you can understand some fan discontent. Either don't mention him every time you're quizzed for goals and let him play in without pressure, or play him more when you clearly need goals. He just looked confused, and it's not like playing Greenwood would have been a bigger risk than playing Pereira, who has been a consistent let down for over a year.