The English Football Association is set to launch an investigation after a Leicester supporter punched a Nottingham Forest player in Monday's FA Cup fourth-round tie.
A fan invaded the pitch after Forest's third goal and appeared to land a blow on Keinan Davis, the Aston Villa loanee, before he was taken away by stewards and arrested by Police.
Arrests at football matches are the highest in years and this latest incident, in the first East Midlands derby since 2014, will only increase concerns over rising fan disorder.
Chief Inspector Neil Williams, of Nottinghamshire Police, said: "We have arrested a man in connection with an incident that took place today during the first half of the Nottingham Forest and Leicester City FA cup match.
"Investigations are in the very early stages and our enquiries into the circumstances surrounding the incident are ongoing."