Ahead of Saturday's visit of Cardiff to Stamford Bridge, Sarri's side sit joint top of the Premier League with four wins from the Italian's first four games in charge.
Chelsea icon Terry captained the club to five Premier League titles, as well as four FA Cups and three EFL Cups, along with a Champions League and Europa League title.
"I'm very happy for him", he continued. "My feeling is that we are not a very solid team in the defensive phase, we need to improve and we can, my feeling during the training is that this team can improve".
The 59-year-old Sarri says "unfortunately at the moment I am not able to stop".
"We have to play seven matches in 23 days, so I had to work", he said.
Chelsea's ideal start to the season has led to suggestions they can challenge for the title after finishing fifth last term, well adrift of champions Manchester City.
"We need another step to be at the same level". I hope not. It's very hard.
"Mentally the match after the break is always very hard".
Mateo Kovacic is once again expected to partner with N'Golo Kante and Jorginho in a midfield that is looking increasingly settled as Maurizio Sarri settles on his 4-3-3 formation.
Ruben Loftus-Cheek is a doubt for the game with an injury, while Emerson and Davide Zappacosta are also unlikely to feature.
"I'm trying to stress this number to my players, just telling them this number, my experience in this situation". He has won everything with the club.' Asked about the possibility of Terry joining Chelsea as a player, Sarri said: 'It would be on the coaching staff, I think, because, as I've said before, we want to give space to the young players.