Despite Trossard's prodigious output and Martinelli's poor run of form scoring and assisting, Arteta keeps going back to starting the latter and trying to play him into form, and going back to using the former as a sub.
If it were absolutely clear the best use of the two players was Trossard starting, Arteta wouldn't be doing that.
We can draw our own conclusions as to why he does it. My guess is that since no one can run that role effectively for 90 minutes due to its athletic demands, starting a gung ho, in-form Martinelli with an in-form Trossard waiting to sub on is a great proposition due to the players' specific traits.
