Lennon stepped down from his role as Celtic manager last month after the Hoops fell behind Glasgow rivals Rangers in the Scottish Premiership title race and could not make up enough ground.
Forrest made his senior debut for Celtic under the Northern Irishman over ten years ago, but was not able to help the manager during his final months at the club as he was sidelined due to an injury.
Having returned to action last week, the Scotland international has expressed his disappointment at not being able to work under Lennon anymore.
Forrest recalled how Lennon promoted him to the Hoops senior squad and had successful spells with the club before going on to wish him all the luck going forward.
“I was gutted, obviously“, Forrest told Celtic TV.
“He was my first manager, he brought me through, had him in the reserves, had a great spell here the first time and then he came back and it started off so well as well.
“I was gutted, the last few months I never got to play under him and that as well I was obviously out injured.
“It has been a hard time for everyone, but obviously gutted to see him go.
“I wish him all the best.“
Forrest made over 190 appearances for Celtic under the management of Lennon.
