The 31-year-old picked up the injury while in action for the Springboks in the Rugby Championship defeat against New Zealand on Saturday and he's now set for a lengthy spell out after tests confirmed the full extent of the damage.
Louw has yet to play for Bath in the Premiership season because of his international commitments and news of his injury setback is a blow to Bath.
"He's picked up a hamstring injury tackling for the ball," Bath first-team coach Darren Edwards told BBC Points West.
"It certainly has an impact on our numbers. With Taulupe Faletau out as well in a similar situation, that's a world-class back row that's not in our team at the moment."