The Reds' lead at the top is just two points after City beat Arsenal 3-1 at the Etihad Stadium.
Victory over the Hammers will be enough for Liverpool to re-establish their five-point advantage, but Pellegrini - a champion with City as a manager in the 2013-14 season - is bidding to throw a spanner in the works.
"If we win I will be very happy for our club first, because we need a good performance against the leaders," the Chilean said.
"After that if we can give a hand to Manchester City, it's not our problem, but of course I am a fan of Manchester City also."
While City are fighting on four fronts this season, Liverpool and Spurs only have the Champions League to distract them from their top fight campaign.
And Pellegrini believes the fixture pile-up for Pep Guardiola's men could prove pivotal to the final outcome.
"It's more easy for the team that is involved in just two competitions compared to the team that is involved in four," he added.
"Tottenham? You should never think that a team four or five points behind you are not involved in the title. Of course they are.
"They have midweek with the Champions League. So it's not easy to compete in all the competitions.
"I talked about this four years ago. The major problem was that we were involved in all the competitions. We won one League Cup in that moment.
"We then went out of the FA Cup in the quarter-final against Wigan. If we'd continued in that FA Cup, I don't think we would have been able to play all the games. We didn't have time to do it."