Valbuena out for up to six weeks
The 29-year-old was forced off in the 73rd minute at the Grand Stade Lille Metropole, leaving the field holding his left shoulder after a collision with LOSC defender Simon Kjaer.
Club doctor Christophe Baudot revealed the extent of the injury on om.net after the match, estimating that the player would be unavailable for between four and six weeks. Valbuena will undergo further tests on Wednesday.
It was an altogether unsatisfactory night for OM, as Nolan Roux's stoppage-time winner secured a 1-0 home win to send second-placed Lille within one point of leaders Paris Saint-Germain.
Marseille coach Elie Baup had been more hopeful over Valbuena's prognosis at his post-match press conference, but acknowledged it was a disappointing night for his side.
"As regards Mathieu Valbuena... if the recovery is good, he will be sidelined for a few weeks - you should speak to the doctor - three or four weeks maybe.
"Conceding a goal in stoppage time, along with our failure to score, and in addition losing Mathieu, and perhaps Dimitri (Payet), who has a calf problem... it's a tough night."
Baup felt his team deserved to get something out of the game - but rued the fine form of LOSC goalkeeper Vincent Enyeama, who collected an 11th straight clean sheet.
"I would say it's unfair," he said. "If you look at the result compared to what happened and what we tried to do, it is really a difficult result to accept.
"We know that Lille, since the beginning of the season, are very good at winning 1-0. They have a quality goalkeeper, who is unbeatable, and he again showed that. In addition, they had this success in stoppage time.
"We should not change our idea of building play and our approach to the game. We must just try to translate all this into goals by improving our finishing."
Baup praised midfielder Florian Thauvin, meanwhile, for not responding to the hostile reception he received from the home fans on his return to his former club following his acrimonious summer switch - and after being the target of an attack when the team landed in Lille on Monday night.
"Florian played the whole game and did not react to anything," Baup said. "Instead, he stay focused on the team, and continued to do his job for the team."