Maybe anceloti should try him at centre-back. 
Against levante yesterday I think there is a hole in that position. Anceloti must dare to experiment. Although I know he is a player with the original left-back position. Or maybe Madrid will buy a new defender? But it looks like that won't be the case for the start of the season.
Alaba's main position is central defender although he can play as left back and defensive midfielder. Ancelotti doesn't need to experiment, it's just that he needs to return him to his main position at centre-back. Real Madrid don't seem to need to buy more defenders because so far they have plenty. Ancelotti has a good plan for his players in the upcoming games and since it is still early then plans to buy new players may not have been considered. Against levante yesterday I think there is a hole in that position. Anceloti must dare to experiment. Although I know he is a player with the original left-back position. Or maybe Madrid will buy a new defender? But it looks like that won't be the case for the start of the season.