Chelsea contacted the police after two messages, described as "totally unacceptable, disgusting and abhorrent" were posted following Wednesday's Champions League draw with Juventus.
The abuse occurred at around the same time as Mikel deleted his Twitter account, although Chelsea insisted the 25-year-old had not been aware of it and had been intending to leave the social media site in any event.
''His intention was to close the account anyway. So it was a good reason to close it," said Di Matteo on Friday. ''I don't think it's fair. If somebody has to express his opinion, he should put his face to it.''