The Danish defender missed Monday's 2-2 draw at Swansea City, having "overstretched trying to prevent a dumbbell falling on his foot," and is now a doubt to face Southampton.
The Denmark international was replaced by debutant Mamadou Sakho for the draw in Wales on Monday after injuring his ribs in the gym and is now a doubt for Saturday's clash against Southampton at Anfield.
"It's too early to say whether he will be fit for Saturday," Rodgers told reporters. "He had a real freak incident in the gym. Just before he was coming out to train, he overstretched trying to prevent a dumbbell falling on his foot.
"He has overstretched and yanked his side. We are waiting to see if that settles down over the next few days. If it doesn't, we have got ample replacements. That is why we brought in extra players."
With Agger sidelined, Sakho made his Liverpool bow at the Liberty Stadium following his 18 million pound move from Paris Saint-Germain on transfer deadline day, and the manager was pleased with his performance.
"Sakho did very well," Rodgers said. "I thought he was excellent and assured. Once he gets used to working with us, he will improve."