AGT: Kenan Thompson to Sub for Simon Cowell as He Recovers from Back Surgery

Kelly Clarkson subbed in for Simon Cowell during the first live shows last week

Kenan Thompson will be filling in for Simon Cowell on America's Got Talent, PEOPLE confirms.

Kelly Clarkson subbed in for the former American Idol judge during the first live shows last week. "Simon's really happy Kelly is standing in for him on AGT this week and grateful to her," a source previously told PEOPLE.

Cowell, 60, broke his back on Aug. 8 at his Malibu, California, home when he fell off an electric bike in front of his 6-year-old son Eric. Hours after the accident, Cowell underwent surgery. He remains in the hospital.

On Sunday, a source told PEOPLE: "He had to have a number of fusions and metal rod put into his back. He landed on his back when he fell from the bike. The injuries are bad but he's also been told he was lucky."

Kenan Thompson. Dimitrios Kambouris/Getty

Simon Cowell. NBC

Two days after the surgery, an insider told PEOPLE that Cowell was "on his feet and walking around again as much as he can," adding, "He's taking it easy but doing well. Simon is already back at work and he's been working on email on his iPad and speaking to a few on his team."

The insider added, "He's definitely not letting this slow down all the things he's got going on."

After the first season 15 live show on Tuesday, AGT judge Heidi Klum spoke with PEOPLE about Cowell's recovery.

"Who knows maybe next week he will be sitting there with us again, at the speed he's going! I hope he does. I was like, 'What else is happening?' Every morning you wake up to another crazy news and I hope this is it for now. Can we just turn the page and can life get better? We're all hoping for that," the model, 47, said.

The live quarterfinal rounds will take place over four weeks and will culminate on Sept. 23, when a winner will be crowned during the scheduled finale.

America's Got Talent airs Tuesdays and Wednesdays (at 8 p.m. ET) on NBC.