Grunge stars Dave Grohl and bassist Krist Novoselic have reportedly enlisted the Beatle to play onstage with them at the Madison Square Garden charity gig.
The Fab Four legend reveals Grohl invited him to "jam with some mates", but admits he had no idea he was filling in for tragic rocker Cobain, who committed suicide in 1994.
Sir Paul tells Britain's The Sun, "I didn't really know who they were. They are saying how good it is to be back together. I said, 'Whoa? You guys haven't played together for all that time? And somebody whispered to me, 'That's Nirvana. You're Kurt.' I couldn't believe it."
The Rolling Stones, Bruce Springsteen, The Who, and Eric Clapton are also on the bill for the 12-12-12 Concert for Sandy Relief.