Rob Smedley admits Williams were "nowhere near" where they should have been in the Monaco GP, their first 2015 race outside the points.
Both Valtteri Bottas and Felipe Massa struggled for pace throughout the grand prix weekend, not necessarily surprising given that Williams have only achieved three Monte Carlo wins.
Neither Bottas nor Masas made it into Q3 on Saturday while they finished a disappointing P14 and P15 respectively on Sunday.
"It was quite a difficult weekend and we were nowhere near where we should be," Smedley conceded.
"At Monaco, if qualifying doesn't go well you have partly made your bed so we knew getting points was going to be tough.
"We have to look at all areas to see what went wrong here with the inherent car performance and set-up. We cannot return here in 2016 and repeat this performance."
However, the team's Head of Vehicle Performance is confident Williams can fight back at the next races, starting with the Canadian GP.
He added: "We now have to focus firmly on the next three races. We have a really good package and some healthy upgrades coming soon so we look forward to coming back stronger in Canada."