The group performed two shows at the music venue in Hertfordshire, south east England, to 150,000 people each night, even though 2.6 million applied for tickets online - meaning the band had enough interest for 20 gigs - and Liam is disappointed the 'Wonderwall' hitmakers didn't do more dates at the time.
Upon being reminded of the news, he told NME magazine: ''Wow, we must have been big. That is absolutely mental. We should have done that, shouldn't we?''
However, the 40-year-old star admits he may not have been at his best had the group decided to extend their run of Knebworth gigs since he completely forgot the band even had a second night to play at the time because he was so hungover.
He added: ''I remember the first night getting completely off me f***ing box with Patsy [Kensit]. I woke up because someone was knocking on the door and saying, 'Come on, you've got to do it all again.'
''I was like, 'What?!' I'd forgot we had another night. Imagine what I'd have been like after 20.''