The striker completed a loan move from Sunderland on deadline day.
The Sunderland striker completed a surprise move to the Ligue 1 side on deadline day, with the French outfit securing him on loan until the end of the season.
His move was arguably the most surprising of the January transfer window deals, seemingly coming out of left-field and catching many unawares.
But Barton has warned the Scotland striker over the move, having spent a year on loan with the club himself during the 2012/13 season.
"There's huge expectation on Steven Fletcher,” he told BBC Sport.
“The club are six points off the Champions League places, so the season is really at a crossroads. It's going to be a baptism of fire.
“It's going to be difficult for him to get in the starting XI. Marseille play 4-2-3-1 and Michy Batshuayi is the main centre forward."
The pressure is certainly on Fletcher to hit the ground running in France, with him not having exactly been in fine goalscoring form of late.
He has only managed four goals in 16 appearances this season for the Black Cats, and only registered five in 30 for them last season.
He will be keen to impress though, with his contract set to expire in the summer and a fine second half of the season in France sure to help him secure a new deal with another club when that happens.