The pair clashed during third-placed Feyenoord's 2-1 home win over the league leaders and their row continued after the game when PSV winger Lens grabbed Mathijsen by the shirt outside the dressing rooms.
Lens, who has nine international caps, received an undisclosed fine from his club on Monday and took full responsibility for the altercation.
"There were several incidents during the match but I should not have waited for him outside the dressing rooms," he told a news conference. "It got several other people involved and they tried to control the situation."
Lens could now be suspended for several matches by the Dutch FA.
PSV coach Dick Advocaat said Lens's behaviour was unacceptable and that he would talk to the player.
"He should have used his aggression during the match," added Advocaat.
Lens and Feyenoord defender Mathijsen are both regulars in Louis van Gaal's Netherlands squad.
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