I completely disagree with that. Dier is a positional player, he marks space until someone enters that space. Wanyama chases players down. You'll see him involved in the press much more than Dier.
The problem is not who takes the man-marking role (if we go that route), it's who takes the second midfield role. The way I see it, we'd have to play Dier screening, and Wanyama man-marking, which means Dembele misses out.
Only one person to do the man marking role and that's Walker. I don't think we need to go down the man marking route