Jenson Button says McLaren was nowhere as strong as expected in Austin despite finishing in sixth place in the United States Grand Prix.
Both Button and Alonso had hopes that the layout and the conditions would suit the McLaren-Honda package to help the team have one of its strongest races of the season.
Although both put one of their best performances of the year, Button said his car's pace was disappointing.
"Our pace was nowhere near what we expected here," said Button.
"So compared to like Toro Rosso, they were so much faster, a lot faster than I expected round here, and especially in the mid-high speed corners I thought we'd be stronger, but we are not.
"So we need to go away and look at why. A tough day, but I'm happy to get sixth place.
"I think we did a good job with that, but apart form that, the pace of the car was not that exciting."
Alonso recovered to run in fifth position after being spun at the start of the race, but ended up losing engine power and dropped outside of the points.
The Spaniard, however, said he had put on his best race performance in years.
"We would have finished fifth and it would have been one of the best races of my career," Alonso said. "And I'm still considering it that, but with a result out of the points.
"I leave with the feeling of having done my best race in several years and hopefully I can repeat it in the three that we have left."