What has Duffer got against Cullen
Duff likes "big men up front", which is why we have Yates, Lowe and Big Myk. Cullen isn't big, so Duff can't see him as a #9. Yates isn't that big in terms of height, but he's more bullish and physical than Cullen. I'm sure Duff wouldn't even play Cullen on the wing if there was anyone with decent speed in the squad to play there instead.
Cullen's an interesting case - he's not big, he's not strong, he's not fast, but he can score. He reads the game well, puts himself in exactly the right places, and can shoot. He's a #9 and really not much of anything else, but his limitations elsewhere on the field should be irrelevant when he can score goals. Every team needs a natural scorer, and Swansea have one. Playing on the right wing. And sometimes in defensive midfield
Duff was also married to the back three when he arrived and he's had to change his mind about that, so maybe there's hope for Cullen yet.