Just before the hour McCarthy, who has been plagued with various injuries, was carried off on a stretcher after nicking the ball off Salomon Rondon in the act of shooting and the Venezuelan's follow-through connected with the midfielder's lower right leg.
McCarthy was stretchered off and was consoled by Irish teammate Seamus Coleman who, incidentally, is nearing a return from a broken leg himself.
Medics rushed onto the field to treat McCarthy, with Rondon in tears having seen the after-effects close up.
West Brom had taken an early lead through Jay Rodriguez's fourth Premier League goal of the season, but Allardyce's side pulled themselves level in the 70th minute through Oumar Niasse's first touch of the game.
With a point in the bag, Everton will now hope that McCarthy can begin the long road to recovery and that it won't be a career-ending injury.
Everton boss Sam Allardyce confirmed after the game that McCarthy had fractured his tibia and fibula, and Rondon took to social media to offer his best wishes.
"He's paid a heavy price for his bravery and commitment".
"Obviously it's a big blow for him and us because he's had so many games missed recently with not-so-serious injuries that we thought we had overcome, and certainly had overcome them over the last few weeks".
Absolutely devastated. Rivalries are forgotten about when a colleague suffers such an unlucky injury. When you see the treatment that a player gets when he gets when injured now, the way forward for him is to get the correct surgeon to heal that leg and really wait until it heals and then fight your way back.
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.
The Irish worldwide has struggled with hamstring injuries over the past 18 months.