De Bruyne came on in the 66th minute and levelled the tie four minutes later before crossing for Sergio Aguero to head home the goal that gave City a 4-3 aggregate victory, setting up a showdown with Liverpool in the final on February 28.
Martinez 'hurt' by Everton's League Cup exit
But as the game approached stoppage time, the Belgian's right knee buckled following a challenge by Everton centre-back Ramiro Funes Mori and after receiving medical attention, he was carried off on a stretcher.
"We'll see tomorrow (Thursday). The doctor thinks, but he cannot be sure now, that he has a problem with his medial ligament. We'll see how hard (bad) it is," Pellegrini told reporters at the Etihad Stadium.
Asked if De Bruyne, 24, might be out for the rest of the season, Pellegrini replied: "I hope not. It's very difficult to know at this moment.
"But I'm optimistic that he will not be out for the rest of the season. It's not the cross (cruciate) ligament, but we'll see how important is the damage."
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