A club statement said Webster, who had been in charge of the Championship club for the last two seasons, had parted company by mutual consent as he had ‘taken the decision to focus on his studies and lecturing work at Leeds Beckett.’
The Australian, 42, led Featherstone to 1895 Cup glory at Wembley in July and they lost just one of their regular league games all season.
Rovers reached the Million Pound Game but ultimately lost out to Toulouse Olympique and saw their hopes of promotion ended once more.
Five-times Grand Final winning coach McDermott - out of a main coaching role since Toronto Wolfpack’s exit from Super League last year - is the frontrunner to now take on the role at Post Office Road.
Webster’s current deal was due to end this month and Featherstone chairman Mark Campbell said: “I had been talking with James throughout the season about extending his contract for the 2022 season so was shocked, when at the 11th hour in the season, he advised me that due to his personal commitments, he didn’t want to coach in Super League if the club were successful in the Grand Final.
“Throughout my leadership of the club, I have had the sole burning ambition of Featherstone Rovers achieving its Super League goal, so I have had no alternative but to decide to look for other alternatives who share the same drive.”
Campbell added: “James’s legacy at the club will be an extremely successful Centenary year where we won the 1895 Cup and came so close to achieving our Super League objective.
“I thank him for all his efforts, we will redouble our efforts to make sure we stand the best possible chance of achieving our goals.”