The pair clashed during Feyenoord's 2-1 win and the row continued after the game when winger Lens grabbed Mathijsen by the shirt outside the dressing rooms.
Len, who has nine caps for the Netherlands, took full responsibility for the altercation, telling a news conference on Monday: "I should not have waited for him outside the dressing rooms."
Other players and officials from both clubs got involved but the FA prosecutor ruled on Wednesday that they had been trying to restore order.
Lens will miss the Dutch cup semi-final against PEC Zwolle later on Wednesday and the next three league matches as he also received an automatic one-match suspension for his fifth yellow card, picked up during the game at Feyenoord.
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