The return of Silva, who last featured for the club in the 6-1 drubbing of Newcastle United on Oct.3, will come as welcome news to City who have scored just three times in their last four league matches.
"David Silva is back on the squad list. It's important to have David back -- he is a very important player, the same as Kun (Sergio Aguero). It's very difficult to replace him," Pellegrini told reporters on Friday.
"David gives us a different way of playing -- we have good possession against teams with eight or nine players of theirs behind the ball and David is the player with the most accurate passes in the final third.
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"Joe Hart has a muscle injury -- you never know as it depends how he improves but I think he will be out for ten days.
"Fortunately it's not a very serious injury for Joe. I never complain about injuries as I trust a lot in Willy Caballero -- I'm sure he will have a good performance," he added.
City, in third place, will look to get back to winning ways in the league after losing 4-1 at home to Liverpool last weekend.
Pellegrini said he expects a difficult encounter against the Saints, who will be without leading goalscorer Graziano Pelle after he picked up his fifth booking of the season in last weekend's loss to Stoke City.
"It wasn't a good week for us -- we lost two games. Against Liverpool it was an ugly defeat and then against Juventus we improved a little bit but it was not the result we wanted," he said.
"Southampton are having a good season, they're unbeaten away and they have good players. We'll need to play well to win.
"They are missing Pelle but (Shane) Long is a very good striker -- they have one suspended player but they will be just as difficult."
(Reporting by Shravanth Vijayakumar in Bengaluru; editing by Justin Palmer)
