let spina play for a whole season and he will make a lot more mistakes, too.
sandro is one of 4 players who played more than 3000 minutes this season. only szczesny, cr and bonucci played more than him. until spina got back, he didn't have any backups. considering the playtime he gets, he's very reliable. i'd find it hard to replace him if he's really gone in the summer.
btw spina is a natural wingback, and can be almost as useful in attack as cancelo, while being a less error prone defender. i'd love to see max experiment with a 4-4-2 with sandy+spina on the left, mds+cancelo or cancelo+berna on the right.