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 the press.
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"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 move from Paris Saint-Germain on transfer deadline day and the manager was pleased with his performance.
"Sakho did very well," Rodgers enthused. "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 that the 23-year-old will develop well 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."