The Hammers had a 1-0 lead at halftime through Jarrod Bowen before Chelsea fought back to win 2-1 through Pedro Neto and an Aaron Wan-Bissaka own goal.
Maresca said afterwards: "It is huge (result). It was a tough game especially mentally. It is never easy when teams come here and wait and sit back. You need to be patient. There is no space, you need to move the ball and wait for the right moment.
"In the second half we came back and overall we deserved to win the game. They were waiting for our mistakes and you can't get frustrated.
"It already happened many times this season that we were losing and came back and won. It is just football. It just depends on the game.
"It is always about the players. It is not about the manager. It was just a feeling we had (over the substitutions) during the game. I think we need to improve in the final third in terms of chances and being more clinical."
On the race for the top four: "We are there and hopefully we can be there until the end. Playing this kind of game in February, where we are still there is an important statement."