Where as he for 60 minutes?
He changed the game imo, even though he was on the pitch through out.
It's quite simple to answer actually. 1st..he played with Rojo at the left. That means Southampton was able to cover him better cause Rojo provided almost no threat going forward. Also, due to the fact that Rojo was having issues at the back Martial needed to track back more which blunted his forward contribution.
2nd...Lingard playing centrally. Lingard provides so little threat that Southampton was able to cover Martial better, same case as with Rojo on LB. If you have time to watch the game again just pay attention on how overloaded their right side of defense was. They were aware that Lingard won't provide much and that Martial is an actual threat and acted accordingly.
Once Pogba was moved further up and Lingard to the right Martial had more room to operate and suddenly we looked far more threatening. It's even more of the case when Mkhi is playing at #10...It's no wonder Martial looks so much better when he actually has some support in areas in with which he connects the most and are crucial for his play,#10 position and LB. Get a proper LB next season (or have Shaw getting back to his best) and Martial will explode. Having in mind how much was he covered and the lack of support he had he did very well.