The 21-year-old Scotland international came on as substitute for Beram Kayal against Ross County at Parkhead just before the hour-mark after recovering from a hamstring complaint and immediately looked back to his best as he grabbed the fourth goal in a 4-0 romp.
The Northern Irishman held his breath when Forrest went down with what appeared to be a reoccurrence before continuing, and Lennon 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."
He added: "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 his in great shape and great condition and we just need to get him on the pitch."
Asked if he might look for an alternative to Forrest in the transfer window, the former Celtic skipper replied: "They are hard to find. What price could you put on a James Forrest?
"He is the best kid I have ever worked with and one of the best kids I've seen in a long time for his age, with his quality and calibre of his play. He comes alive and makes people get off their seats at times so to have him back at this stage of the season is great.
"He brings that pace to the team and opens teams up. We were stodgy at times in the first-half because we don't have that natural width but he gives us that."
County boss Derek Adams was left ruing chances his side missed in the first-half, and said: "It was disappointing because in the first-half we had a number of very good opportunities to take the lead but we didn't take them.
"You don't get many opportunities when you come to a venue like this."