Fosu-Mensah joined the London club on loan from United at the start of the season, as the Cottagers looked to bolster their squad to ensure Premier League survival.
Despite his lack of involvement it's believed Fosu-Mensah enjoyed his time at Fulham.
In an apology to fans on Twitter he said: “I want to apologise to all the amazing fans and say I'm sorry I couldn't help keep the club up this season.
“We did all we could but it just wasn't enough. It's a fantastic club and I know you will be back in the premier league very soon."