Roma coach Eusebio Di Francesco said Kevin Strootman is discussing terms with Marseille and could move to the club this week.
The Dutch midfielder is expected to undergo a medical in the French city on Monday, with Di Francesco leaving him out of his squad to face Atalanta that same evening.
A fee reported to be in the region of €25 million has been reached and Di Francesco confirmed talks were advancing.
"There are negotiations underway for him to be sold and it would make no sense for me to sit here and try to hide things," he said at a news conference in Rome. "I've not yet spoken to the player, but I will do so first thing this afternoon.
"You know how I see things -- while he is still a Roma player, as was the case with [Edin] Dzeko [last January], I will continue to consider him as such. But certainly for me, Roma must come first and then the individuals, even if we're talking about a great player like Strootman."
Strootman joined Roma from PSV Eindhoven five years ago and was being linked with a move to Manchester United in 2014 before a serious knee injury forced him to miss almost two years.
The 28-year-old has made 102 Serie A appearances and is set to be reunited with former Roma coach Rudi Garcia, who said his arrival would be "good news" following his side's 2-2 draw with Rennes on Sunday.
"We've got to be prudent when we're in the middle of a transfer window, but we'll see if it goes through or not -- if it does, it will be good news," he said at a news conference.