I dont really give a toss about what Keane thinks.
Take a look at this video and pay attention to Martial (I know, its Bruno highlights, but its all i could find)
The positions he takes up are so weird for a LCF. Either hes standing still right in front of one or two CB's which makes him super easy to mark, or he has drifted out wide and is hugging the touchline. Like at the clip at 5:09. Unless hes been instructed too (which i find unlikely). What on earth is he doing out there when there is acres of space down the middle and no one there? Another one at 5:22. Lightly jogging in front of the defense. That's incredibly easy to deal with for the defenders
Another one at 5:33 when Bruno jumps over the ball and Greenwood jumps over the ball. To be fair to Martial, he busts a gut here to get forward, but his starting position is just weird. He has drifted wide (again), but is about 5 meters behind their offside line. Because he is rapid as feck, he gets on the ball in the end and takes a shot, but had his positioning been better here this could have been a goal
And this is not an isolated example. When he (Martial) is poor, him barely getting involved is a very common theme. Martials physical and technical attributes are top notch, his positioning, movement and (arguably) his work ethic are awful at times