He was discussed as a left back since he played there in youth teams. He broke through as a winger but his skillset is everything you want in a modern fullback. He was crushing it, we have wingers, and Tierney is further behind in terms of fitness and integration in the squad. Less said about Kolasinac the better.
Playing him anywhere else at this stage is stupid, and the last two games bear that out, especially if it's not even at LW! Maybe he'd become a good midfielder in 2 or 3 years, but he's so young it seems ridiculous to crowbar him in to the worst Arsenal midfield for decades.