The Uruguay international suffered a dead leg in training and the thigh muscle later swelled up, causing him to spend time in hospital.
Saints manager Nigel Adkins will not take any chances with his £12m summer capture and the 21-year-old midfielder looks likely to miss their next four matches.
Ramirez has also been ruled out of his country's 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," said Adkins.
"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."