Published Friday, 30 March 2012
Stiliyan Petrov of Aston Villa during a Barclays Premier League match (© Getty)
The Aston Villa captain was diagnosed with the condition after he developed a fever following last Saturday's defeat to Arsenal.
The 32-year-old Bulgaria international is a former Celtic player - and Lennon has expressed his "absolute devastation" at the news.
Petrov - affectionately known as 'Stan' - was signed for Celtic by John Barnes and spent seven years and won four Scottish Premier League titles for the club.
He added: "Stiliyan has the heart of a lion and as part of the Celtic family he deserves all our best wishes and support".
Support also poured in for Petrov from his Villa teammates on Twitter.
Goalkeeper Shay Given tweeted: "In shock about Stiliyan Petrov acute leukaemia but we are all here for him and his family. Please God he makes a full recovery.#prayforstan."
A statement from the club said: "We expect to learn more about Stiliyan's situation in due course and we have moved quickly to support him and his family. During this time we ask that Stiliyan's privacy is placed ahead of all inquiries and trust that we will share information as we receive it."
The statement continued: "Stiliyan is cherished by many and he will get from Villa every ounce of love and support that we have to help bring this to a positive conclusion."
Petrov was handed the Villa captaincy by former manager Martin O'Neill, under whom he had previously excelled at Celtic.
He established himself as a key figure at Villa Park and in 2008-09 Petrov was named players' player and supporters' player of the season. He also remains a firm favourite in his native Bulgaria, for whom he has won over 100 senior caps.