The X Factor judge was scheduled to join John Mayer, Ne-Yo and The Roots at Vh1's Philly 4th of July Jam in Pennsylvania, but she recently fell sick and on Tuesday (02Jul13) admitted she was suffering with a fever and swollen tonsils, forcing her to pull out of the gig.
Now Hayes has stepped in to serve as Lovato's replacement while she recovers.
Event organiser Desiree Peterkin-Bell says, "We are very excited to have Hunter Hayes join us for the Philly 4th of July Jam tomorrow... Hayes is one of the biggest stars in country music right now and we know that he will be a great addition to the stellar lineup we already have in place!"
Alerting fans to the news in a post on Twitter.com on Wednesday (03Jul13), the 21 year old wrote, "Just Announced! @HunterHayes is performing Live at #PhillyJam Tomorrow! Be sure to Tune In".