2nd base is easier to play than SS and by all accounts he was at least a decent defensive shortstop. He also played on the 2B side of the diamond on shifts quite a bit, so it's not like he hasn't fielded a ball on that side of the field before. If he were to play 2B full-time he can learn all the intricacies of the position in ST.
The question with Andrus isn't whether or not he can play 2B defensively, it's whether he'll hit better than the 75 wRC+ he put up in the years prior to last season.