Jordan Pickford will undergo a scan on Monday to assess the extent of the rib injury that forced him out of Everton's game with Burnley.
Pickford lasted until just before half-time at Goodison Park on Saturday before having to be replaced by Joao Virginia, who made his Premier League debut.
The Blues goalkeeper missed three league games in February with the same rib problem and Everton will perform tests on Monday to determine the extent of the re-occurrence.
"We don't know," Ancelotti said, when asked how long he expected to be without Pickford.
"He has exactly the same problem as the month when he was out for two or three games.
"He has a scan on Monday and we will see after the scan.
"I hope that Jordan can recover soon but we have to see, we have to check.
"It is not going to be a big, big problem because it is the same problem [as before].
"It is true he was out for two to three games but I hope he can recover soon."
Robin Olsen was again missing through injury which meant Ancelotti had to call upon Virginia.
Academy goalkeeper Harry Tyrer was also on the Everton bench on Saturday night.
