Frankie Valli ’s fans are expressing concern for the 90-year-old’s health after he appeared weak and struggling to lip-sync at a recent concert, but a rep for Valli exclusively tells The Post he’s “just fine.” A video of the legendary singer’s difficult-to-watch performance has already been viewed more than 4.1 million times.

Recorded at Valli’s August 3 gig at Mountain Winery in the San Jose suburb of Saratoga, the video shows the singer looking frail as he fights to climb a short flight of stairs to the stage and gripping a handrail as he does. Valli got a warm welcome from the crowd, whom he paused to acknowledge before finally making it to the stage. As his set gets underway, the crooner appears to be lip-syncing to his 1978 hit song “Grease,” which was also the movie’s opening credits track.

He seems to struggle to mouth the words, and his face wanes. As the video plays on, the “Can’t Keep My Eyes Off Of You” singer snaps his fingers slowly while holding a mic somewhat weakly as he’s flanked by backup vocalists. However, a rep for Valli has told The Post on Tuesday: “Frankie is doing just fine and super happy to still be performing.

The audiences are filling venues and listening to some great music.” “Frankie is doing what he loves to do at 90,” they added. “We should all be so lucky.

” The video, which kicked everything off, was posted by Instagram user @cecchinitrucking on August 3. “Great show!!!! Frankie shows his age but at 9.