The Chilean says the veteran midfielder must "fight for a position" within the side until he moves to America in January to prepare for the new MLS season
Goal learned on Friday that the England midfielder was set to sign a short-term deal with the Premier League champions as part of their partnership with the new MLS outfit.
And following the Blues' penalty shoot-out defeat to Olympiakos, Pellegrini revealed the midfielder will join the club prior to the start of the Premier League season.
"As you know Frank has signed a contract with New York City and will start working here in January as the season starts in March," he told reporters.
|MAN CITY LATEST
|9/4||Man City are 9/4 with bet365 to retain the Premier League title|
"At the moment he will work with the squad and be a member of the squad. He will fight for a position.
"He knows we have a very good team, we have a lot of players in his position. He is one player more.
"We have a lot of games particularly with the Champions League - so he's very important for our team."
Reports have suggested that Lampard would refuse to play against Chelsea when his new side face Jose Mourinho's team in the 2014-15 season.
But Pellegrini is hopeful that the player will be available for every match and plans to discuss Lampard's availability with the veteran midfielder.
"I will talk about all those things with him but he'll be a player for City and he'll play every game he needs to play.
"Is Lampard a six month loan? For the moment he will start working with our team from now until January and we'll see after that.
"He's a player for New York City and we'll what happens when he comes to us."
Lampard could make his first appearance for Manchester City when they face Arsenal in the Community Shield on August 10.