Lennon won a second consecutive treble as Celtic manager on Sunday when his side beat Hearts to win the Scottish Cup at Hampden Park.
Celtic won a fourth straight treble, but their performances in the league remain the big problem and Rangers are now 16 points clear of the Bhoys in the league table, while playing three games more.
McManus believes that the cup win has done little to alleviate the pressure on Lennon and the calls for his sacking will return as soon as Celtic fail to win a game in the league.
He insisted that Lennon has to win every game until the Glasgow derby on 2nd January, but stressed that if he loses that one he could be out of the door at Celtic regardless of what he has achieved as manager.
The former Hibernian forward said on PLZ Soccer: “I think he is on a knife-edge still.
“The supporters are obviously delighted with the result and you saw a lot of them outside the Hampden, celebrating, but I think he is one result away or a draw away in the league from them wanting his head again and throwing things at the telly.
“It is still in the balance and he has got to win every game.
“I think they have got a pretty easy run-in, to be honest, some easy games before they play against Rangers at Ibrox.
“That game is going to be absolutely crucial for Neil Lennon.
“The board gave him the cup final and I think they are going to give him until that game at Ibrox on 2nd January.
“If Rangers beat Celtic he will be out of the door and the league is gone.
“I think the league is gone anyway, but that will just cement it.
“He has got to win every game, including that Rangers game.”
Lennon will need his side to take all three points when Celtic host Ross County at Parkhead on Wednesday evening.
