Williams is expected to start tonight against Champions League opponents Ajax.
Robertson told the club's website: "I think Neco has come in for unfair criticism but, look, unfortunately that is the nature of the beast. It's going to happen when you do make one mistake or whatever it is.
"But Neco is a confident lad and the way he has played for Wales in the previous international breaks has been unbelievable and he shows promise and he shows glimpses of it. It's just about him getting that complete performance. I believe it's coming and I believe it's close because he's full of quality and he's full of passion. You can see that the way he flies into tackles and stuff.
"He's a great player with great quality and through Trent's injury he's got quite a few opportunities and he's done well. He knows he can do better and that's why we will try to push him and he'll try to push himself."