The 22-year old goalkeeper suffered the injury earlier in November when he collided with a goalpost in the 1-1 draw with QPR and will now undergo surgery to fix the trouble.
"On route back home from London, was expecting the outcome with the specialist. Surgery on Monday. My season over! #Raging," McCarthy posted on Twitter.
McCarthy stepped into Brian McDermott's first team on seven occasions in the Premier League when the form of Royals number one Adam Federici had waned.