Koeman was always stressing about how big the jump was from the Dutch league to the Premier League and how you have to be really careful in picking which players will work and which won't.
With Pelle he obviously thought the fact he's a big unit with a good touch would help that transition but with Klaassan nothing he did for Ajax stood out as what we may need. He always looked a player on the fringes of games who would pop up with a goal or an assist after 90 minutes, which is clearly a nice attribute to have but is very reliant on others around you and not your own individual qualities.
He's suffering, as they all are, for playing in a side devoid of structure, confidence and quality throughout.