The usually dependable Kompany endured an indifferent campaign as City finished a distant second to Chelsea in the Barclays Premier League last term.
City conceded an average of a goal a game and Pellegrini wants to see improvement this time around, but he is confident the experienced Kompany will lead the way.
Pellegrini, whose side begin at West Brom on Monday, said: "I trust a lot in Vincent.
"It is always important to analyse why we concede 38 goals and more than 35 per cent were from set-pieces
So it is important to concentrate on the second balls and not to concede so many free-kicks or corners.
"I don't think defending comes from just one player - it comes from the team
Last season was also a good experience for Vincent and I am sure we will see the player we saw playing so well two years ago."
Defensive issues have remained a problem throughout pre-season, with City losing 4-1 to Real Madrid a fortnight ago and then conceding another four before half-time against Stuttgart last weekend.
Pellegrini insists the matter was not serious last season, but acknowledges the back line must tighten up.
The Chilean said: "I think it is not good to divide a team into defence and attack - the team is the team.
"You have 11 players, you can play in any tactical system you want, but the 11 players know how to attack and the 11 players must know how to defend in the way we play.
"We were not a disasters in defence last season, but I think 38 goals conceded in 38 games is too much
I think a team that wants to win a title must concede not more than 30 or 31 goals."
Pellegrini, who has signed a contract extension to 2017, has rarely deviated from his preferred attacking style during his first two seasons at the Etihad Stadium.
That has attracted criticism but, again, Pellegrini insists he has no intention of changing his philosophies.
He proved doubters wrong by winning two trophies in 2014 and insists last season's shortcomings were more down to a lack of concentration than a fault with the system.
He said: "The style we have in this club will continue in the same way you have seen this team play in the last two seasons
It is not fortunate or lucky to be the team that scores the most goals.
"We didn't win the title, but we continued being an attacking team
Maybe we needed to be a more competitive team or play with more concentration in some games that maybe we thought were easy games
You never play an easy game
We lost so many points against the teams fighting relegation.
"There are many things we expect to improve this season, but no change in style of play because I am absolutely sure it is the best way to win."
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Source: PA