Red Bull Racing looked distinctly off the pace after the opening day of practice at Monaco on Thursday.
Mark Webber finished fifth 0.7 seconds slower than the fastest time of the day set by Mercedes driver Nico Rosberg, while championship leader Sebastian Vettel finished ninth more than 1.3 seconds down.
“My run on super-soft was very poor and we need to understand why,” he said. “I wasn’t able to improve and we should be able to quite easily so we need to have a look. After that I did a long-run on super-softs and it seemed quite okay.
“All in all it was okay, but unfortunately we had to make a big change and we lost quite a lot of time in the afternoon.
“It’s not completely ideal but that’s Friday - or in this case Thursday. We’d like to be more competitive in low-fuel configuration, so we need to try and find some lap time there.”
Webber too seemed perplexed by the RB9’s lack of pace, but noted that Saturday’s qualifying session was the most important time to be quick.
He said: “I think traditionally we’re never too electric here on Thursday’s, so today was actually one of the better ones we’ve had in the last few years.
“You’ve got to be very, very focused here; concentration is very, very high. It’s a challenging venue to pull everything together; we want to obviously get short run performance as well as gaining short and long run data, which I think we’ve got.
“There are some quick cars out there; we’ve seen that on Saturday at the last few GPs. All in all, a reasonably smooth day for us, we’ve got plenty to go through now.”