Well, firstly, I don't think he's actually made many 'costly mistakes' for Brighton.Have you noticed the costly mistakes Dunk has made of late, I’m not sure RDZ has control over that.
Secondly, if 'mistakes' are made (for example) through taking risks in dangerous areas, or leaving defenders hopelessly exposed then clearly the manager does 'have (some) control' over that.
And lastly, if a manager doesn't have control over a player's form, who does?