Police are investigating a new assault allegation made by a patron of the Sydney pub where a Penrith rugby league player was allegedly involved in a brawl on Saturday night.

Penrith NRL club stood down hooker Masada Iosefa for one week on Monday after he was charged by police with affray, resisting arrest and failing to leave a licensed premises on Saturday night.

The club said 21-year-old Iosefa had indicated he would "vigorously defend" the charges.

On Tuesday, a man told Macquarie Radio his grandson suffered facial injuries in an assault outside the Mean Fiddler Hotel about 11.30pm on Saturday night.

He confirmed his grandson believed the man was a Penrith NRL player, and said the assault occurred prior to the alleged affray at the same hotel.

"They pulled up outside in a cab, there were two girls and two young fellas, and (my grandson) was getting out of the cab when he got hit," he said.

"He (the player) was just out of control. Then they watched him go up and take to the security guard."

He said his grandson had made a statement to police.

When asked about the new assault allegation, police would only confirm a complaint was being investigated.

"Castle Hill Police are investigating a complaint about an assault at a licensed premises on Windsor Road on Saturday night," a NSW police statement said.

"A 21-year-old Kings Langley man has told police he was assaulted by an unknown man as he was entering a licensed premises after getting out of a taxi some time between 11 and 11.30pm.

"He received facial injuries as a result.

"An initial report was made last night by the alleged victim.

"At this early stage of inquiries police believe the victim does not know the identity of the assailant."

Police are reviewing CCTV footage of the incident and have appealed for any witness to contact Crime Stoppers on 1800 333 000.

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