Conte told Costa via text that the striker was no longer in his Chelsea plans two weeks ago.
The 28-year-old, who had an up and down relationship with Conte at last season, is now in uncertain about his future as he tries to find a new club.
Manchester United legend Cantona believes Conte is wrong to dump the Spanish striker and believes the Italian’s actions could cost Chelsea.
Cantona said: “Angry texting, it’s not a good idea and breaking up with a text it’s even worse.
“Mix the two together and you have Antonio Conte’s now infamous: ‘Good luck for next year but you are not part of my plan.’
“Every single letter of that sentence addressed to Diego Costa may cost Chelsea about a million euro.
“Don’t run a team with your thumbs!”
See Also
Paul Merson: ‘Chelsea need to fill these two positions’
Didier Drogba: ‘This star could be heading to Chelsea’
Antonio Conte responds to claims Victor Moses cheated