It's only a matter of time before he's back as a #9 for us. With Young back, Memphis needing games and hopefully bound to regain some form eventually, Martial will be back as a striker for us with Rooney dropping deeper. We might bring someone like Mane in January as well. I think it's quite certain that we'll bring a wide player either in January or next summer.
I think Van Gaal knows that Martial is better as a striker, but had to play him on the left because of circumstances such as Young's injury and Memphis's form. He's great out wide, but most of the times he's far from the goal hugging the touchline, and we have no one in the box. There was a moment yesterday when Rojo crossed the ball into their box, but there wasn't anyone on the receiving end despite it being a great cross.
Yes, he has great technique, but he is also a very composed finisher and doesn't get bullied by the CB's. It's not like he doesn't get the chance to run at the opposition when he's playing up top. He constantly drifts out wide on the left. That's how he scored against Liverpool after all.
We should sign someone like Mane or Bale or if we sign a striker, it should be someone like Aubameyang so that he and Martial could interchange positions.