The two sides meet at the London Stadium with West Ham boosted by ending a four-match losing streak following their first win of the campaign against Everton last weekend.
Chelsea, meanwhile, have been in irrepressible form under new manager Maurizio Sarri, moving to the summit of the table with five victories in as many matches, and kicking off their assault on the Europa League with a 1-0 win over Greek side PAOK on Thursday.
"I know that here in London there are a lot of derbies, but we have our first of the season against Chelsea, and they are at the top of the table so that is more of a motivation to beat them," Pellegrini said.
"It is impossible to start better than Chelsea have, but every game is different so we must be confident that we can play, and we need to be very concentrated on defending because they have players that make a difference in space.
"But, if we beat Chelsea it will not change our season. We will have just had six games, and not 38, so if we lose it will not be the final decision either. We must continue to try and improve every game, and every week."
Pellegrini finally claimed his first league win as West Ham manager with an encouraging 3-1 defeat of Everton at Goodison Park.
Declan Rice, deputising for Jack Wilshere, who faces six weeks on the sidelines following ankle surgery, was one of six changes by Pellegrini at the Toffees following West Ham's dismal defeat at home against Wolves.
The 19-year-old impressed as the anchor in West Ham's midfield, and it is likely he will get another chance to build on his performance against the Blues.
And Rice will face a stern test when he comes up against France's World Cup winner, N'Golo Kante.
"Players always have an opportunity to play, and when they have the chance they must take that chance," Pellegrini added.
"Declan did very well in the last game. He is a very young player, and he must know a lot of different things, learning about different players, and of course Kante is one midfielder that must be an example to him.
"Chelsea is a team that moves the ball very quickly, so we are going to try to do the same thing, and of course the midfield have a very important role in that."