The Italy international was substituted 30 minutes into the match and is expected to be sidelined for just about the remainder of the season after the club revealed the extent of the injury.
"We have tested Antonio Cassano following the accident which occured during the first half of Inter-Atalanta, and results of his scan showed a second degree lesion myotendinous to his right hamstring," Inter Milan wrote in a team statement.
The news comes less than a week after striker Rodrigo Palacio was ruled out for the same duration, and, with Diego Milito also in the treatment room, it leaves Inter coach Andrea Stramaccioni with Tomasso Rocchi as the only available center forward for the visit of Roma in the second leg of the Coppa Italia semifinal next Wednesday. Roma carries a 2-1 lead into the return leg at San Siro.
Rocchi netted his 100th goal in Serie A - his first in more than a year - in Sunday's defeat, but speaking to the club's official website, the 35-year-old expressed regret that he could not celebrate reaching the milestone with a victory.
"I hadn’t scored for a long time, and I needed to regain the right mindset to face matches and get my form back. But I can’t be entirely happy. It would have been better to celebrate my 100th goal with a win," Rocchi said.