Australian ace Latham was forced off late in the Guinness Premiership match with Newcastle Falcons and initial reports suggested only bruising to his knee.
However, following further medical analysis, Ruddock admits Latham, who has been in inspired form in recent weeks, is now set for an enforced spell on the sidelines.
"When we looked at him originally we thought it would just be bruising, he limped off and the medical staff were quite upbeat," said Ruddock.
"Unfortunately when they had a second look at him they noticed more movement in the knee which indicated there could be some ligament damage.
"He has had an initial scan which confirms there is some ligament damage and he is going to see a specialist on Friday to get a comprehensive view on it.
"The early indicators are that it is four weeks and a medical ligament strain. Unfortunately that means it looks like he won't be back in time for the Bristol game.
"Having said that, if there was ever anyone to beat the medical timescale it is Chris Latham so we will see what happens."