"I feared the worst on Sunday and it has been confirmed that Matthew has ruptured his Achilles," said Blues boss Steve Bruce.
"Of course, we are keeping our fingers crossed he can return as soon as possible but you are looking at six months out.
"It is terrible news, a real blow for us and the player and he is booked in to have surgery on Tuesday.
"We had eased Matthew back carefully after his other injuries and had hoped he would be available for the finale to the season.
"Over the last three-and-a-half years he has been a big, big player for us in establishing Birmingham in the Premiership and being given his England chance.
"I have always maintained he is the best naturally left-sided central defender in the Premiership and he has been immense for us.
"Our thoughts are with him now but he is a strong character and let's hope he can get back as quickly as possible."