Rosenior, who replaced Enzo Maresca at Stamford Bridge earlier this month, had made an impressive start, winning his first four matches in charge across all competitions.
Chelsea also boosted confidence in midweek by coming from behind to beat Napoli away and secure a place in the Champions League last 16.
The Blues sat fifth in the Premier League, just one point behind fourth-placed Manchester United, but Rosenior warned his players against complacency ahead of Saturday's clash. Chelsea also faced a crucial League Cup semi-final second leg at Arsenal on Tuesday, where they needed to overturn a 3-2 deficit to reach the final.
Speaking ahead of the West Ham game, Rosenior stressed the importance of consistency after the victory in Italy.
"For me tomorrow was the biggest game of the season," he said. "It's very simple, I was going to find out so much about the group. We're in a good place, but if you want to be a consistent winning team, these are the games where you have to step forward.
"After a big win in Italy, we had to come back three days later and show the same focus. I wanted to see the consistency levels of the group. That's why this game was a massive marker of where we really were."
Chelsea faced a West Ham side showing signs of improvement under Nuno Espirito Santo, even though they remained in the relegation zone despite back-to-back wins.
Rosenior praised his opposite number, saying: "I'm not surprised Nuno has put his stamp on the team. His teams are always very well organised without the ball, whether it's a 4-4-2 or a 5-4-1 shape.
"And they've always been very good in transition moments."
With momentum building and major fixtures ahead, Rosenior believed the derby would show whether Chelsea were ready to take the next step.
Arne Slot has urged Liverpool to cut out mistakes as they prepare for a decisive Premier League clash with Newcastle United, with the Reds boss admitting the race for the top four has reached a critical stage.
Mikel Arteta has called on his Arsenal players to stay calm and confident as they prepare for a tricky Premier League trip to Leeds United, insisting the title race will not be defined by a short dip in results.
Michael Carrick has urged his Manchester United team to be wary ahead of the Premier League clash against Fulham despite a flying start in his role as interim head coach.
David Moyes has admitted Everton were bracing themselves for a major setback after confirming Jack Grealish was likely to miss the rest of the season with a stress fracture in his foot that needs surgery.
