The 23-year-old was forced off after 33 minutes against Crystal Palace U21s last month.
Pilgrims boss Tom Cleverley said the club will take extra precautions due to Oseni’s previous ankle problems.
The Nigeria international joined Plymouth from St Mirren last summer and impressed with 10 goals in 33 appearances last season, earning his senior Nigeria debut in May.
“Because it’s an ankle he’s injured in the past, the specialist has advised we take a little bit more time to make sure that ankle heals and is as stable as it can be,” Cleverley told BBC Radio Devon.
“I’m disappointed for him because his season’s not got going.
“He missed a large part of pre-season, and then he’s trying to chase a little bit of fitness. Then, in a sort of nothing action of the game, chasing a ball into the channel, the defender falls on him and his ankle sprains.
“It was an important game that he played in against Palace Under-21s, and unfortunately he didn’t come out the other end of it fit.”