The England international left Chelsea when his contract ran out at the end of last season and in July joined new Major League Soccer (MLS) club New York City on a two-year deal.
New York City are owned by Manchester City and Major League Baseball's New York Yankees and will play at Yankee Stadium when the 2015 MLS season gets underway in February.
"As you know Frank has signed a contract with New York City and will start working here in January," Pellegrini told a news conference after his side's pre-season friendly with Olympiakos Piraeus in the United States.
"From next Wednesday he will start working with our team and will play for us until January. 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," said Pellegrini.
"He is one player more. We have a lot of games particularly with the Champions League so he's very important for our team."
Manchester City play Chelsea, the club Lampard served for 13 years, in the league on Sept. 21 and there have been media reports that the player would prefer not to feature in the game at the Etihad Stadium.
"I will talk about all those things with him but he'll be a player for Manchester City and he'll play every game he needs to play," said Pellegrini.
The 36-year-old Lampard is Chelsea's record scorer, having hit 211 goals in all competitions after moving to Stamford Bridge from West Ham United in 2001.