Roberts was left with a cut lip following the incident in Greyfriars Road in the early hours of Sunday.
The Cardiff Blues centre, 24, was on a night out with fellow Wales player Jonathan Davies at the time.
A 20-year-old man from the Caerau area of Cardiff was arrested and released on bail.
A spokesperson for South Wales Police said: "A 20-year-old man from Caerau, Cardiff, was arrested on suspicion of assault following an incident, which saw a man obtain minor facial injuries, on Greyfriars Road at 0320 on Sunday, 20 February.
"He has been bailed and released pending further inquiries."
The Welsh Rugby Union refused to comment.
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