Di Maria was on the left last season and had his best season. I believe Januzaj is best on the right as it makes better use of his left foot. A lot of left footers prefer the right side because they can cut inside and passing with the inside of their left foot occurs more often cutting from the right. For example, Ronaldo, Bale, Ribery, and Robben. All play on opposite flanks of their preferred foot.
This is what Ozil said, "It’s different playing on the left. When I was in Madrid, I often played on the right. I enjoyed that because I’m left-footed and I was able to cut inside to give assists and get shots on target. On the left, it’s more difficult. If I do get past someone I’m still away from the danger zone, still on the flank, and it’s harder for me to find the final ball with my right foot than with my left."
Di Maria is one of those players that can operate on both flanks.
I know pre-season results mean nothing but the players looked like they knew the system, we were scoring plenty and creating a lot of chances as well as playing some really nice football at times. It's like last season is still haunting the players and there just like "holy shit we're back in the PL again" and the arses have dropped right out of them...bizarre, although injuries are playing a big part in this imo. 
