It's been an injury-plagued season for McCarthy, who was playing only his fourth Premier League game of 2017-18 due to successive knee and hamstring problems.
Medics rushed onto the field to treat McCarthy with Rondon in tears having seen the after-effects close up and Seamus Coleman, only just recovered from a serious double fracture of the leg himself, was seen leaving his seat in the directors' box apparently to be with his Republic of Ireland teammate.
Things went from bad to worse for Everton, who fielded new signing Theo Walcott from Arsenal, when 27-year-old McCarthy sustained his injury after the interval as he tried to prevent West Brom's Salomon Rondon from shooting.
Asked how soon the player would have his operation he added: "When we do it will depend on the specialist surgeon".
Rondon was about to pull the trigger when McCarthy nipped in to tackle him and the striker kicked McCarthy instead.
The Toffees' official account tweeted shortly after accident: "Heart-breaking moment for James McCarthy as he is stretchered off with what looks like a broken leg after a last-ditch challenge".
"We had just worked hard to get him back to fitness".
Everton have confirmed that Ireland midfielder has suffered a double leg fracture.
The Glasgow-born central midfielder has been with Everton since 2013 when he joined for £13m from Wigan Athletic, linking up with former manager Roberto Martinez.