The versatile academy graduate reunited with former Ajax youth coach Frank de Boer in south London earlier this month on a season-long deal after United strengthened the spine of their team with the respective additions of Victor Lindelof and Nemanja Matic.
However, despite being dispatched to Selhurst Park to increase his exposure to regular first-team football, Fosu-Mensah still appears to be part of Mourinho's plans for the future.
In October, he was handed a new four-year contract that includes the option of a further year and, according to The Sun, is worth a total of £5.2m ($6.7m) with wages of approximately £25,000-a-week.
"He's been texting but sometimes he rings," the 19-year-old told The Mail of his constant contact with Mourinho. "Sometimes it is the assistant too. It's definitely good to have that contact because I'm still under contract there.
"He's told me to just keep focusing on my game. We spoke and both came to the agreement that this is the best move for my development. He's said that we should keep talking to each other."
Netherlands Under-21 international Fosu-Mensah, playing as part of a new-look back three, endured a 78-minute competitive debut to forget for Palace as De Boer's side slumped to a shock 3-0 home defeat to newly-promoted Huddersfield Town. However, he still demonstrated his formidable speed with a superb recovery tackle on Steve Mounie that prevented the impressive Benin forward from notching a hat-trick.
He played the entirety of Saturday's (19 August) fixture on Merseyside, where the more defensively resilient Eagles' run of three successive wins at Anfield was ended courtesy of a second-half effort from Sadio Mane that secured a 1-0 triumph for profligate Liverpool.
Capable of being deployed at right-back, centre-back or in a holding midfield role, Fosu-Mensah, who joined United in 2014 aged 16, made his senior Red Devils debut against Arsenal in February 2016 and featured on nine further occasions under Louis van Gaal. He was restricted to just 11 appearances during Mourinho's maiden campaign at Old Trafford, only four of which were starts.
"Tim is young player with great potential," Mourinho enthused last year. "I am pleased that he has signed a new contract and I am delighted at the progress he has made so far. It is clear to see that he is learning every day in training."