3 'sitters' is a ridiculous assessment of his chances since none of them were. At what point do we actually count the goalkeeper in the equation? I'm really curious which 3 chances you saw that you'd consider sitters, because I'm watching the highlights and not one of them qualifies as such. The only close one would be the pass from Pogba at the edge of the box, but right after the first touch the GK makes a great rush out and blocks the shot, it's just great goalkeeping and not a sitter. That volley was from a tight angle with the keeper well placed so, again, not a sitter. Sterling's miss against Lyon was a sitter, just so you have a reference of what it actually means.
And yes, he was let down because it was pretty much up to him on his own to create danger in the final third, Rashford was awful and looked disinterested, while Greenwood was fairly quiet and couldn't bring much to the game, neither of them made up for their lack of offense by creating danger or space for Martial and others to work into.