He is also a really good presser of the ball.
Issue is, we only ever (or have done anyway) press in one's or two's, rather than as a whole team.
So Klaassen will drive forward, press the oppo, but they'll be nobody backing him up, which means there's a simple pass on for them and Klaassen looks like a div and there's now a huge gap where there's no players.
You have to press as a unit. It's key to being successful.
Look at City. They win the ball back with such speed and ferocity, because there's always three players around the ball at any time.