The 20-year-old, who only signed a short-term deal last Friday, was sentenced to a 12-month community order at Harrogate Magistrates Court on Thursday, narrowly avoiding jail.
The Millers have now acted by suspending him, pending an investigation.
A club statement read: "The club can announce that following recent reported events in the press and having spoken with the player concerned, it has decided to suspend Simon Lenighan pending an investigation into the matter.
"The club will make a further statement as and when appropriate but will not do so whilst the matter is under investigation.
"The club hopes to finalise its investigation in a speedy manner and will make further comment as and when appropriate to do so."
The court heard how Lenighan headbutted Leah Davey outside a nightclub in February. He avoided prison, but was handed a 12-month community order and ordered to pay court costs and damages to Miss Davey.
Lenighan was unlikely to be involved in Saturday's Championship clash with Wolves prior to his suspension.
