The 36-year-old, who spent 13 years at Chelsea, recently signed for New York, but as the next Major League Soccer season does not start until next year, he has opted to return to England to maintain fitness.
New York are one of Manchester City’s two sister clubs, the other being Australian side Melbourne City.
“As you know, Frank has signed a contract with New York City and will start working here (the United States) in January as the season starts in March,” Pellegrini told a press conference.
“From next Wednesday he will start working with our team and will play for us until then.
“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 and 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.”
Pellegrini was speaking after City lost on penalties to Greek side Olympiakos in a friendly in Minneapolis late on Saturday.
Lampard has a strong emotional attachment to Chelsea, having become the club’s all-time leading scorer with 211 goals, and Pellegrini suggested he could be spared from facing his former club when Jose Mourinho’s side visit the Etihad Stadium on September 21.
“I will talk about those things with him, but he is a player for Manchester City,” said the Chilean.
“He will play every game that he needs to play.”
Lampard’s move echoes that of fellow New York new boy David Villa, the Spain striker, who has joined Melbourne City on a short-term loan deal.
The return to the Premier League could help Lampard extend his international career, which has seen him win 106 caps and score 29 goals for England.
Lampard is City’s fourth close-season signing, after Brazilian holding midfielder Fernando, French right-back Bacary Sagna, and Spanish goalkeeper Willy Caballero.
City face FA Cup holders Arsenal in the Community Shield next weekend before opening the defence of their Premier League title away to Newcastle United on August 16.