However, the Bhoys youngsters frustrated their illustrious opponents for much of an entertaining game at the BME Stadium.
The Parkhead side were still trying to come to terms with playing on Astroturf when their Portuguese opponents took the lead after just three minutes.
Benfica broke down the right with Nelson Oliviera getting past Andy Hinkel and hitting the post before Keirrison pounced on the loose ball and slotted home.
Despite the setback, Tony Mowbray's side responded with Danny Fox testing the Benfica keeper and Paddy McCourt relentlessly running at the defence.
Paul McGowan chased every ball up front and four seconds into injury time at the end of the first half, the 21-year-old got his reward.
McCourt caused problems for Benfica allowing Mark Wilson to get in a shot which hit the post, with Bellshill-born McGowan outmuscling his marker to slot home.
McCourt then hit the bar from almost 30 yards as Celtic attacked with purpose in what had become a close-fought game.
However, 13 minutes into the second half and Celtic were undone when Hinkel was caught in possession with Ruben Pinto firing the ball past goalkeeper Dominic Cervi.
Javier Saviola then hit the post before the same player wrapped up victory a quarter of an hour from time when he tapped home after Cervi failed to hold on to a shot.