Everton have stated that they are prepared to issue severe punishments to any fan who is found guilty of throwing objects at Chelsea midfielder Ross Barkley during Sunday's 2-0 victory.
The 25-year-old, who graduated from the Toffees' youth academy, drew frustration from many fans after opting to take a cut-price move to Stamford Bridge in January 2018. He was loudly booed during Sunday's match, and a coin was also allegedly thrown at Barkley during the first half.
A club statement read: "We are investigating the alleged incident and reviewing footage. If anyone is found responsible for this or other such incidents, the club will take robust action."
Until he was substituted after around 60 minutes of the game, Barkley was subject to incredibly vocal jeers and taunts by the Everton supporters, many of whom will not have forgotten about the circumstances of his departure from Goodison Park in 2018.
The England midfielder was close to completing a move away from the club in the summer of 2017, but pulled out of the deal at the last minute, instead preferring to evaluate his options as he recovered from a hamstring injury.
He did not appear for the club again, before completing a £15m move to Chelsea in January, which was far cheaper than the initial deal for Barkley in the previous summer.
The incident came just a matter of days after the FA, Premier League and EFL issued a joint statement condemning any crowd unrest, in the wake of the pitch invasions affecting both Jack Grealish and Chris Smalling.
On the EFL website, they wrote: "English football has worked hard over many years to ensure that our grounds are safe places. Everyone involved in staging a game deserves to enjoy the highest safety standards and we are determined to prevent these standards from being undermined.
"The FA, Premier League and EFL wholeheartedly promote the Football Supporters' Federation's "passion not poison" call to respect opposition Clubs and fans. We encourage all supporters to carefully consider their behaviour, not only at this weekend's Premier League, EFL and FA Cup matches, but at all games in the future."