Uruguay midfielder Ramirez suffered a dead leg in training, but the thigh muscle later swelled up and he was forced to spend time in hospital to ensure the problem didn't get any worse.
Saints manager Nigel Adkins has no intention of rushing the £12 million ($18.81 million) signing from Bologna back into action too quickly and so Ramirez looks likely to miss their next four matches.
Ramirez, 21, has also been ruled out of his country's 2014 FIFA World Cup qualifiers against Argentina and Bolivia.
"The muscle swelled up very much because there was nowhere for the blood to go, so there was a lot of pressure on his thigh," Adkins said.
"He has had to have a couple of days in hospital to try and relieve the pressure, so obviously he is not going to go and join the Uruguayan team over the international break."