The Hoops were due to find out this afternoon who they will meet in the last four-clash at Hampden – with Dundee United, Hibs and Falkirk all in the mix.
But Forster's wonder save from a Graham Carey free-kick ensured Celtic held on for a 2-1 quarter-final win over st Mirren – and broke the Buddies' hearts.
Lennon said: "He is brilliant.He got a big hand to it and he has done that for us many times this season.
"Graham is very good at that. He (Forster) was aware of what was coming, he made a great save and we recovered very well.
"But by that time I did feel that we should have been out of sight. I think we deserved to win the game, especially with our first-half showing."
Lennon is optimistic that the victory gives Celtic a lift prior to their second-leg Champions League last 16 tie against Juventus.
"Psychologically, Saturday will give them a big lift going into this game and it's very much one to look forward to.
"To go there and win would be a tremendous way to end the campaign. We want to make it look like the scoreline at Celtic Park was a little bit flattering towards Juventus.
"We felt there wasn't much between the teams although they do have a lot of class in their side.
"We'd like to go and compete, and try to win the tie."
Celtic are facing a monumental task to try to recsue the tie.
And Hoops fans hero Lubo Moravcik admitted his old club would need to harness the spirit of their famous win over Barcelona to cause a huge upset against the Seria A leaders.
He said: "I was lucky enough to be there when they beat Barcelona on matchday four.
"I hoped they would keep a clean sheet against Juventus, because then they would have a chance of going through.
"They conceded three goals, so now it's mission impossible. In football there are surprises, but not miracles.
"The night they beat Barcelona was quite something. It was extraordinary. It's the sort of thing that happens once every 10 years, but they did it.
"The people were so proud, you saw Rod Stewart crying. It's a country with wonderful people and they love football.
"I'm really happy that I was able to experience my time with Celtic, because my career wasn't complete without that. I love Celtic."