The 28-year-old suffered the injury last Saturday during his side's 1-1 draw with Southend United in League Two.
Hoskins signed a one-year deal with Oxford earlier this summer following his departure from Brighton & Hove Albion.
"It's obviously a big blow to us and to the player himself," United manager Michael Appleton is quoted as saying by BBC Sport. "Will had started to look sharp and was linking things together at the top of the pitch for us."
Oxford continue their League Two campaign on Saturday when they travel to Exeter City.