Trossard hit the floor in the 90th minute after colliding with Southampton's Nathan Redmond and required lengthy treatment on the pitch before he was stretchered off.
Early indications are that the Belgian international suffered some nerve damage to the right elbow and Albion head coach Graham Potter was reluctant to provide a timescale on the injury.
"It was his elbow," said Potter. "I don't think it is as bad as we initially thought. I have not had a massive amount of information. The early diagnosis is a little more positive than we first feared. We will just have to see how he goes over the next day or so."
The injury to Trossard saw Albion go down to 10 men as Brighton had used all the substitutes to chase the game.
It's the second time in two matches Brighton have been reduced to 10 following Adam Lallana's thigh injury at West Ham.
Remarkably it's also the second time Albion have forced their way back to a draw with 10 men in the dying moments.
Neal Maupay once again fired home in injury time for his sixth of the season. It's the third time this term the Frenchman has struck to claim a 1-1 draw for Albion. He did so at Crystal, West Ham and now Southampton.
"He does not give up and he has a great mentality," said Potter. "He keeps working and believes in himself and believes that another chance will come.
"He fights for the team. the harder you work the luckier you get. If it keeps happening you can say it's something other luck.
"He's resilient and always waits for the next opportunity."
Brighton will now rest and recover ahead of Tottenhan this Saturday at the Amex but the injuries are piling. Trossard is clearly a doubt while Sarmiento (hamstring), Lallana (thigh), Adam Webster (calf) Lewis Dunk (knee), Danny Welbeck (hamstring) and Steven Alzate (ankle) are all out while Shane Duffy will be suspended.
"It was his elbow," said Potter. "I don't think it is as bad as we initially thought. I have not had a massive amount of information. The early diagnosis is a little more positive than we first feared. We will just have to see how he goes over the next day or so."
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