The Englishman has been linked with a move to the Portuguese side, but Celtic are in no mood to negotiate any kind of deal - and have penned a letter to Benfica warning them against trying to make a move for Forster.
Having already lost the spine of last season's side with Gary Hooper, Victor Wanyama and Kelvin Wilson all gone, the last thing Lenny wants is to lose his No.1 keeper who played out of his skin last season.
While Benfica may still tempt the club with an offer, Lennon has warned that Celtic will rebuff all interest.
"No," said Lennon. "Fraser's not for sale."
Forster has yet to rediscover his form of last season and Lennon believes that the player's focus will be fully on Celtic once the window closes and he is still in Glasgow.
"I think it will be once the window shuts," said the Hoops boss when asked whether or not the player's head was in the right place.
"He's got a lot to look forward to - but he's not for sale. We had no confirmed bids for him.
"We have actually written a letter to one club, just warning them off basically."
While Lennon has added Finnish striker Teemu Pukki and Israeli midfielder Nir Biton, he would still like to add at least one or two more before tonight when the window closes.
The Celtic manager had been irked in the immediate aftermath on Wednesday night that he hadn't been given "the help he needed", hinting a degree of discord between himself and the board where money and signings have been concerned this summer.
However, he has conceded that he believes everything that can be done has been done, although he is not without frustration at the protracted dealings in the transfer market.
"I think there's money there to spend it's just about trying to get the right player in for the right price," he said.
"We're always looking for value for money. There have been players where we thought, 'Yeah' and the asking price has gone up and I've even pulled the plug on one or two because I didn't think they were worth the money.
"I would have liked, like everybody else, to have done the business earlier and quicker but that's just not the case sometimes."
Lennon added: "We do our business cleanly and simply and quickly, but sometimes that can't be the case."