The midfielder broke into the England squad last season under Slot.
The Dutchman, ahead of tonight's Premier League opener with Bournemouth, told talkSPORT: “I think if you ask him and if you ask me, we both have the same idea - that's as a number eight. That would be his best position. So with a number six next to him or behind him and then he has a bit of freedom.
“He played in every single position and if you're able to do that, you feel comfortable everywhere on the pitch. So if you feel comfortable everywhere on the pitch, you like to go to different positions.
“For me, a number six should be much more about keeping your position. So he can do this, but he prefers to have a little bit more freedom.
“As a No.10, he's also able to play that role, but I think he's better if the ball is in front of him or he's behind the ball. Let's put it that way.
“When he has to wait for the ball to come to him because he's quite impatient, which is also his quality. He likes to have the ball a lot. So for me, his best position would be the number eight. And that's also probably the position where we have most competition with.”