The booing with Rashford obviously wasn't for the player, but the fans feeling frustrated and wanting to see both Martial and Rashford in the pitch at the same time. I think it's something we would all love to see from time to time, and considering the game was still in the balance the frustration was understandable.
As for the Lukaku one, you here those sorts of groans all the time in these very tight games. A stray pass, a missed mass, a player holding on to the ball too long. I think Lukaku could have got the ball, but just hesitated slightly. Nothing to do with him being lazy at all, just kind of misjudged the speed of the ball i think.
Frankly i don't see anything wrong with it. The fans obviously support and like him. Jose bringing that up was a little silly and unnecessary.