The Italian also said Ashley Cole was back in light training after breaking his left ankle at Everton at the start of February and he expected the England defender to return to the first team before the end of the season.
Czech Republic keeper Cech has been out since he was carried off on a stretcher with a calf injury in the Champions League last-16 first leg match at Inter Milan on February 24.
Third-choice keeper Ross Turnbull, who has played the last three games, will drop to the bench against Portsmouth. Ancelotti also said he expected second-choice Henrique Hilario to be fit again for Saturday's home game against Aston Villa.
Serbia defender Ivanovic, who has played at full back for most of the season, limped off with knee ligament damage midway through Sunday's 1-1 draw at Blackburn Rovers.
"Ivanovic is an important player for us," Ancelotti said. "He is out for one month but we hope to recover him before this period.
"Fortunately we have Yuri Zhirkov recovered so we can put Paulo Ferreira at right-back and Zhirkov at left-back."
Chelsea, who have 65 points from 30 matches, have a game in hand on Manchester United and can move within one point of the Premier League leaders by winning on Wednesday.
"The mood was not good after the Blackburn game because when we don't win we are disappointed," said Ancelotti, who believes a points tally of 86 will win the title.
"But we have a possibility to come back (strong) tomorrow."
