Fulham had a busy transfer Deadline Day, as Fosu-Mensah finalised his switch to the club alongside the likes of Joe Bryan, Luciano Vietto, and Sergio Rico.
The 20-year-old featured in the pre-season for the Red Devils, but has now moved to the Premier League newcomers for the 2018-19 season.
Fosu-Mensah spent the 2017-18 campaign on loan at Crystal Palace, and made 24 appearances in all competitions for the Eagles before returning to United.
"A good loan is a loan to play", Mourinho told MUTV on Thursday morning.More news: Google Pixel 3 XL fully revealed in hands-on pics and video
"We are looking to build on the hard work and results achieved last season and I believe this is the most important part of making that happen".
Goalkeeper Fabri is touted to make his first start, while Callum Chambers will make his debut at the heart of the defence.
VIETTO first signed with Atletico in 2015 (he was at Villarreal beforehand) where he was later loaned out to Sevilla and Valencia.
Fosu-Mensah spoke to the club's official website following the confirmation of the switch, stating: "I have a good feeling about the club".
He did manage a hat trick against Las Palmas in the Copa del Rey in January, and now will be seeking more consistent form as he moves to London.