Clarence Chandran,



chief operating officer, will take a six- to 12-month medical leave of absence, the company announced Tuesday.

Chandran was seriously injured in a 1997 stabbing attack in Singapore. In recent months, he has had surgery twice to address complications from the attack. Chandran's doctor advised him to take a medical leave so he could fully recover, according to a company statement.

John Roth, president of Nortel, will assume Chandran's daily duties.

Chandran has also been nominated as a member of Nortel's board of directors. He stands for election to the board of the Canadian telecom on April 26. Roth said he expects Chandran to be able to serve if elected.

In recent trading on the

New York Stock Exchange

, Nortel fell 25 cents, or 1.5%, to $16.40.