Koeman, who lost Seamus Coleman, Ramiro Funes Mori and McCarthy to injury over the international break, was specifically upset over McCarthy's hamstring injury, criticizing O'Neill and Ireland for how it handled his player. O'Neill fired back with an eye-opening response, calling Koeman a "master tactician of the blame game" and imploring him to stay silent.
"Once again the Everton manager, master tactician of the blame game, has struck out in his comments today, criticising both myself and James McCarthy. Perhaps a review of Everton's pre-season programme might provide some enlightenment.
"James had a magnificent tournament for the Republic of Ireland last summer during UEFA Euro 2016, playing his last game in very late June. He then returned to Everton after a very short break, but only 11 days later, he played his first of three games, all within an eight-day period, against Real Betis, Manchester United and Espanyol. Overloading?
"It should be added that James last played for his country on the October 9, 2016-almost half a year ago. Since that time he has been totally under Everton's supervision. James is diligent and conscientious in his professional preparation. Perhaps, in this instance, quiet introspection may serve the Everton manager and his medical staff better."
Koeman fired back from his personal Twitter account following O'Neill's statement:
James McCarthy began his pre-season three and a half weeks after Ireland were knocked out of the Euros. From the master tactician
- Ronald Koeman (@RonaldKoeman) March 31, 2017
Everton returns to action on Saturday in a Merseyside derby against Liverpool, which also had misfortune over the international break with the loss of Adam Lallana to injury.