The incident occurred after Watford's 2-2 draw at Wolves, when a 44-year-old man was attacked and taken to hospital with head injuries.
His condition is critical but stable.
A statement from West Midlands police on Thursday confirmed a 13-year-old boy had been arrested, while an 18-year-old, who was arrested on Wednesday, has been released on bail.
The 18-year-old must report to a police station on a daily basis, and is banned from Wolverhampton city centre on matchdays.
An online fundraising page has been set up by a Wolves fan in aid of the injured Watford supporter - named on the site as Nick Cruwys - and donations have since passed the £25,000 mark.
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