Lennon has had some memorable midfield battles with Paul Hartley

Hartley linked up with Lennon at the Celtic Park when he signed from Hearts in January, and says the club captain's absence will leave a gap in the squad.

"It's a bit of a shock, I only found out this morning that Neil was leaving," said Hartley.

"It's his decision, but we hope he can leave with two trophies and go out on a high."

Interview: Celtic midfielder Paul Hartley

Lennon announced his intention to leave Celtic when his contract expires at the end of the 2006-2007 season.

"He's been a terrific servant for this club," said Hartley. "He will be very difficult to replace. He was a terrific player for Celtic and I've had a few good battles with him."

"In the four months I've been here, I've learnt a lot from him and he'll be a big miss."

He was a terrific player for Celtic and I've had a few good battles with him." Paul Hartley

Celtic chief executive Peter Lawwell also singled out Lennon for special praise, and said the Northern Irishman would be remembered by fans as one of Celtic's greats.

"Without doubt, Neil Lennon will rightly be regarded as one of the club's finest servants.

"Neil has served Celtic with distinction for nearly seven years and has achieved the highest of honours - SPL titles, Scottish Cups, League Cups, not to mention a European final and some famous Champions League victories.

"Clearly, we are all disappointed to see Neil leave the club, but it is important that we celebrate his contribution and all that he did in bringing such success to the club in recent years.

"On behalf of the Celtic Board I would like to sincerely thank Neil for his magnificent contribution to the club.