The Denmark international was replaced by Mamadou Sakho for the draw in Wales on Monday, after injuring his ribs in the gym.
And Rodgers has revealed that Agger is also in doubt for Saturday's clash against Southampton at Anfield.
"It's too early to say whether he will be fit for Saturday," Rodgers said.
"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 debut at the Liberty Stadium following his £18million move from Paris Saint-Germain on transfer deadline day.
Rodgers was pleased with the defender's contribution and has backed him to improve.
"Sakho did very well," he explained.
"I thought he was excellent and assured. Once he gets used to working with us, he will improve."
Sakho's manager at PSG last season, Carlo Ancelotti, also feels the 23-year-old will get better as a player in England.
The current Real Madrid boss told L'Equipe: "I exchanged SMS (messages) with Mamadou Sakho when he was transferred to Liverpool and I wished him good luck.
"This is a championship which suits him."
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