Tomlin was taken off after just 11 minutes of Posh's Capital One Cup defeat at the Stadium of Light last week following a challenge by the Black Cats' Ki Sung Yueng.
Scans showed the 24-year-old had damaged ligaments in his foot and ankle and he missed the Sky Bet League One victory over Rotherham on Saturday.
Speaking after that game, manager Darren Ferguson said: "He'll be out for four weeks - perhaps longer.
"He's a young boy and we like to think he's a quick healer.
"He'll be out for four to six weeks but we've got Paul Taylor for six weeks on loan from Ipswich, and others we can use."
Tomlin has scored seven goals in 11 appearances for Peterborough this season and was a summer transfer target for Scottish champions Celtic, who had three bids for the forward rejected.