Mignolet is currently playing for Belgian champions Club Brugge.
“Simon is just a very good goalkeeper," Achterberg told HLN.
“Because of that negativity, nobody came to him, even though he wanted to play every week."
Achterberg continued: “Not one day did I ever think that Simon was not doing his job. He is so professional that he knows how to leave it behind.
“How do you do that? Keep working and go back to basics. Doing things as simply as possible. With full concentration, without taking many risks. Trying to get back into your good graces."