Think Ole's managing him perfectly. It might be frustrating for him to go off in the last fifteen mins but it's the right thing to do. Workrate has never been his strongest issue and it's okay as he has other great qualities.
I agree that he definitely doesn't have the endurance yet to play 90mins in our side currently - but at the same time he's the only player we have who can hold up the ball.
Almost every time Ole has substituted him to try and 'manage' our lead, there's no one who the defence/midfield can pass to while we transition from defence to attack - this has pretty much led us to conceding goals due to our inability to get out of our own half.
I think Greenwood will be able to do this role in the long term, but I can understand why we've been linked to experienced target men like Mandzukic in the short term.
For now it's up to Martial to improve his own endurance & fitness, it probably wont get to the level it needs to be at this season and if he keeps getting niggling injuries it will continue to hamper his progress too.
Obviously the most ideal situation would be to have a comfortable 3/4 goal lead in every game