The Woking squad endured a difficult start to the season in the first two races as its car proved to be far from the pace of the frontrunners.
Ninth places from Jenson Button in Melbourne and Sergio Perez at Sepang are its best results to date.
But Whitmarsh is optimistic that hard work during the break will help McLaren be more competitive when Formula 1 action resumes at Shanghai.
"Clearly, our performance in Australia and Malaysia fell below the high standards we've come to expect at McLaren," said Whitmarsh.
"However, the three-week break between rounds two and three has been a useful one; we have been fully engaged in further developing our understanding of the MP4-28 and are confident of being able to implement a number of changes this weekend that will widen its operating window and, potentially, improve its performance."
Button, who felt he could have finished on the podium at Sepang, says the Chinese GP will be like a new start to the season for him.
"In a funny way, the Chinese Grand Prix almost feels like something of a reboot of the start of my season," he said. "Taking home two points from the first two races obviously wasn't what we had in mind at the beginning of the year, but I think everybody in the team has picked themselves up and really attacked the task of addressing our car's issues.
"We know where we're losing performance, so I think we're actually all looking forward to seeing just what we can achieve in Shanghai.
"The important thing for us as a team is to go into the weekend with clear focus, hone our strengths across the sessions and increase our understanding of the car. I definitely think there's everything to play for with this championship, and I can't wait to get back into the cockpit to start the fight again."
- Sports & Recreation
- Motor Racing
- Jenson Button