Singer/songwriter Jack Johnson has stepped in to replace Mumford & Sons as the headlining act at the Bonnaroo music festival in Tennessee on Saturday (15Jun13), after the folk band was forced to pull the plug on its performance to allow bassist TED DWANE to recover from brain surgery. The I Will Wait hitmakers had to scrap a series of gigs this week (begs10Jun13) when Dwane was hospitalised with a blood clot on his brain. He has since been discharged after undergoing surgery, but the group has had to pull out of the festival gig. Their decision left Bonnaroo organisers scrambling for a replacement, but on Friday (14Jun13) they announced Johnson had agreed to hit the stage at short notice. However, the Better Together hitmaker admits he has some practice to do before the big performance. He says, "I've got a lot of lyrics and chords to relearn by Saturday night. I was here to play the first Bonnaroo and it is a very special festival to my band. We are excited to hit the stage again." Johnson warmed up for Saturday's show by performing a surprise set in the Bonnaroo media tent on Friday (14Jun13), when he told reporters: "So the circumstances are pretty unfortunate. I was really looking forward to seeing Mumford & Sons!" Johnson joins a line-up which already boasts the likes of Paul McCartney, Tom Petty & The Heartbreakers, Bjork and R. Kelly, although rapper Earl Sweatshirt has had to axe his appearance at the four-day festival after falling ill with pneumonia. According to Billboard.com, country star Darius Rucker had also been approached to fill in for Mumford & Sons, but his tour schedule didn't allow for a festival appearance on Saturday.