The Dons were winning 10-6 when Mennell suffered a serious kneecap injury, which looks like it has brought the curtain down on his career.
Featherstone won 28-16 in the end, with Doncaster disrupted by Mennell’s exit.
“I think that will be it for Paul,” said head coach Gary Thornton, according to the Doncaster Free Press.
“Paul’s a tough lad so when you hear him cry out like that then you know it’s serious.”
The defeat to Fev confirmed the Dons’ relegation to League 1 next season, but Thornton, who was recently confirmed as the club’s coach for 2016, has already started the recruitment process for next season’s squad.
“Already myself and Carl Hall [club chief executive] have been doing a lot in preparation, we have a squad of players who are here right now we will continue monitoring these players to decide on who we want to keep and build on,” he said.
“There will be some people staying, I have my own ideas too on players I want to bring to the club and also how I want us to play.
“We are starting afresh now, we draw a line under this year and we will bring players in to put a team together make us competitive in League 1.”