Benzema limped off with half an hour played at Celtic Park after feeling discomfort in his right knee and was replaced by Eden Hazard, who went on to contribute a goal and an assist.
"It doesn't seem worrying. We'll have to wait tomorrow for tests," Ancelotti said in his post-match news conference, when asked about the extent of Benzema's injury "The first evaluation doesn't seem very serious, but we'll have to wait for tomorrow."
"[Benzema] felt something in his knee," goalkeeper Thibaut Courtois told Movistar. "We'll see, I hope it isn't serious. If it is, there are other players who can come in."
Vinicius Junior opened the scoring in the 56th minute of the group F opener before Hazard played in Luka Modric to double Madrid's lead and then the Belgian wrapped up the scoring himself, dispatching Dani Carvajal's cut back.
Madrid opted not to bring in a centre forward during this summer's transfer window.
"I thought this was a game for Eden," Ancelotti said on Tuesday. "His position, a bit deeper, could help us... He was clinical, we scored the second thanks to him and he was in the box to score the third. A perfect game for him."
"It's important for [Hazard] to feel good," Courtois said. "Eden's a great player ... Once he got up to the speed of the game he was important for us."
"It's never been a question of quality [with Hazard]," midfielder Toni Kroos added. "It's been his physical condition, his injuries. Today he came on and did well, let's hope he carries on like that."
Madrid have won all six of their games so far this season, lifting the UEFA Super Cup against Eintracht Frankfurt and taking four straight wins in LaLiga before Tuesday's victory at Celtic.