Manager Bryan Robson said: "I've been disappointed with his attitude at a time when other midfield players have moved above him in the pecking order.
"The defensive side of his game isn't good enough as far as the type of midfield players I want at this club are concerned."
Koumas, 25, who cost £2.5m from Tranmere in August 2002 has been inconsistent during his time in the Midlands, making 94 appearances, scoring just 14 times.