Cowell is said to have cancelled the contract of Ali Tennant and re-hired Yvie Burnett - who had been let go after the last series - as well as appointing Savan Kotecha as the new lead vocal coach.
A source told The Sun: "The new coaches weren't cutting it. The groups have a lot of complicated routines and weren't gelling at all.
"There are only three days until the live shows but when Simon saw the shambles he fired the new coaches and bought back Yvie. Now the singers only have a few hours each with her to get it right. The show is in chaos."
However, another insider called Tennant's sacking "totally unfair", adding: "There's not much he can do in a week if the acts can't sing."
Tennant claimed that the decision came as a shock and expressed his disappointment that he had been given no explanation for the axe.
His manager Alex Field said: "Ali proved popular not only with the contestants but also with the judges. He was the only vocal coach to be invited by Simon Cowell to his house in Malaga for the selection stage.
"He was extremely surprised his contract was terminated without notice."
Tennant has previously worked with artists including Alexandra Burke, JLS, Westlife and Boyzone.