The singer, 57, is renowned for his larger-than-life stage persona and his big-ego live shows, but in recent months has been seen to add more conversation to the set, seemingly to give his singing voice more downtime.
The issue appeared to come to a head in Denver, Colorado last night when Coverdale stopped the show after four songs, admitted to the crowd he couldn't go on, and walked off stage.
And there are now rumors the band will not continue their US tour with Judas Priest.
Witness J Morton reports:"He looked fine but sounded awful. Honestly, I've never been so concerned for his future. Even after his many pauses I knew he was coming back - but this is something different. I foresee a tour cancellation."
Another fan says: "He indicated his voice wasn't hacking it. It was a graceful exit considering the circumstances. He really did give it the 'old college try' but it was no use. It was really sad to watch."
But there are also hopes the singer is simply unwell, and got back on stage too quickly after suffering an attack of laryngitis. Whitesnake cancelled their scheduled show in Arizona last Friday after doctors advised Coverdale not to sing. At the time the band stated they had no intention of cancelling further gigs.
Morton adds: "David couldn't hit anything mid-range and he was very croaky. He may legitimately be ill. The sad part was you could really see him trying to make it through."