The 21-year-old Scotland international came on against Ross County just before the hour-mark after recovering from a hamstring complaint, which had kept him out since October, and immediately looked back to his best as he grabbed the fourth goal in a 4-0 romp.
The Northern Irishman admitted he held his breath when Forrest - whom he described as "the best kid I ever worked with" - went down with what appeared to be a reoccurrence of the same injury before continuing.
Lennon stressed the esteem in which he holds Forrest when he said: "It was a huge bonus to see James back. We were able to go to our more natural formation when he comes on.
"He felt his glute (gluteus maximus) towards the end so we are keeping an eye on that but it is great to have him back.
"If we lost him for a length of time again it would be devastating for us because of the wonderful talent that he is.
"He knows he has to report any extra niggles straight away so we are going to monitor him very carefully and make sure his is fit and ready for the games.
"He has worked really hard the last six, seven weeks and you can see he's in great shape and great condition and we just need to get him on the pitch."
Celtic skipper Scott Brown struck a minute after the interval to break the Highlanders' resistance and Gary Hooper helped himself to two classic poacher's goals before Forrest grabbed the fourth with a fine solo goal.
Lennon added: "I am delighted, thrilled with the performance, it was a good home win which was coming."