In the old days, specialist full backs had one task: to deal with wingers. Force them oustide, and then block the cross, usually with was what was then called a sliding tackle. Our own Charlie Aitken was an exemplar of the trade.
Now, a winger is a luxury, and just needs pace plus one trick. Bolassie has prodigious pace plus 4 or 5 tricks. He is lethal in this division.
The downside is that Everton will want him back.