Ancelotti is well aware of Alonso's influence having been impressed by the Spaniard's pairing with Javier Mascherano during his former club AC Milan's UEFA Champions League final clash with Liverpool in 2007.
Alonso's return to Spain has seen Lucas Leiva step into midfield alongside Mascherano, but the Brazilian has yet to convince in the role.
And, ahead of Liverpool's visit to Stamford Bridge on Sunday, live on Sky Sports 1 & HD1, Ancelotti does not believe the Reds' engine room is as strong as it was.
"The midfield of Liverpool put a lot of pressure on you because Alonso and Mascherano were fantastic players," said Ancelotti.
"Lucas is a young player and he has to get experience. I think he will be a very good midfielder, but without Alonso, they have lost something."
Not healthyMeanwhile, Lucas believes the focus of Liverpool's ups and downs should be on the whole team, and not just star duo Fernando Torres and Steven Gerrard.
"I don't think it is healthy that some people blame individuals when the team is losing," said Lucas in the Sunday Mirror.
"Everyone knows Gerrard and Torres are world-class players who are very important to Liverpool.
"But Liverpool aren't about individuals. We should not be about blaming certain people. If we win, we do it because we are a team. If we lose, it is exactly the same.
"My belief is that individuals can only play well if the whole team is playing well and that includes Stevie and Nando.
"If we are going to beat Chelsea it is because the whole team is playing well. Hopefully we can all play well at Stamford Bridge and that will allow our big players to show their quality."