Fletcher, a four-time North East derby winner, joined the French giants on loan until the end of the season in a surprise deadline day move and he made his debut from the subs bench in Sunday's 2-1 defeat at home to Paris Saint-Germain.
And afterwards Fletcher - who scored in the 3-0 win over Newcastle at the Stadium of Light back in October - revealed the atmosphere at the Stade Velodrome was like nothing he had experienced in football before.
"The atmosphere was fantastic," the Scotland striker told French website Le Phoceen.
"It is like nothing that I’ve seen in the past, and it was good to see the fans like this from the pitch."
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Zlatan Ibrahimovic put PSG 1-0 up after just two minutes before on-loan Newcastle midfielder Remy Cabella equalised for Marseille.
But ex-Manchester United ace Angel di Maria netted the winner with his ninth league goal this season and the win moved PSG 24 points ahead of second-placed Monaco.
Fletcher's Marseille, meanwhile, are down in 10th and five points adrift of the Champions League places.
The Sunderland loanee added: "In the end, we're obviously disappointed because we played well, especially in the second half.
"We need to give the same type of performance in the next game, which will be very important."
The 28-year-old, whose Sunderland contract expires in the summer, had not appeared for the Black Cats in the Premier League during 2016.
His exit leaves Jermain Defoe and on-loan Trabzonspor man Dame N’Doye as the only orthodox strikers available to manager Sam Allardyce.
But Allardyce is adamant he has enough firepower in his ranks, with widemen Fabio Borini, Duncan Watmore and £9million January capture Wahbi Khazri all capable of playing in a central role.
Allardyce told the Echo: "He hadn’t played as regularly recently as I expected him to.
"So I felt that it was the right decision to let him go to Marseille, rather than keeping him here."