Sessegnon has been on the radar of a host of top-flight clubs since making his debut for Slavisa Jokanovic's side last season and has seen his stock rise significantly during the current campaign, scoring 12 goals and providing four assists to help his boyhood club situate themselves in the play-off spots.
Fulham batted away interest from United and Tottenham in January and will hope to enhance their chances of keeping the left-back-cum-winger by securing promotion to the Premier League, a division they have striving to get back into for the last four seasons. But Rosenior, who was voted Fulham's Player of the Year in 1987, thinks a summer deal for Sessegnon has already been agreed with a top-flight side.
"I think that a deal has been done for Ryan as teams have been in for him," Rosenior said on Monday's edition of the Totally Football Show.
"It's a Premier League side with strong rumours of Tottenham. I wouldn't be surprised if other sides have been in for him as well. He stays at Fulham before moving in the summer. His future is already mapped out.
"Ryan has great potential. He is one of those players who you think he's not a left-back and not a left-winger. He could be an amazing player."
It remains to be seen if Rosenior's claims actually materialise but Fulham will no doubt find it extremely hard to keep hold of Sessegnon, who doesn't turn 18 until May, if they don't secure promotion back to the top-flight.
The England youth international has two-and-a-half years remaining on his contract at Craven Cottage, and Fulham have recently evidenced their resolute nature when interest emerges in their coveted stars.
The powers that be in west London abruptly warned West Ham United away from captain Tom Cairney in the January transfer window, insisting that not even £40m would tempt them into selling the former Blackburn Rovers midfielder, per Sky Sports, while they also rejected a £25m bid from Tottenham for Sessegnon in the summer transfer window.
Sessegnon is certainly not desperate to leave Fulham anytime soon and is solely focused on achieving promotion back to the Premier League whether it be through the automatic places or the play-offs, but Fulham's resolve may prove fallible if they remain in the Championship for another campaign.
Jokanovic's men demanded £50m for Sessegnon in the summer, according to The Times, and they will hope to command a similar fee should they decide to part with their lauded academy graduate.