The Germany international believes the Gunners were playing well up until when they conceded the first goal, but says they must maintain a high level during their performances
The 28-year-old centre-back believes the side were strong in the opening half hour of the match, but says they were punished by goals from Gareth Bale and Aaron Lennon after losing focus.
"We are disappointed because we looked good for the first 35 minutes," Mertesacker told reporters.
"We'd been solid at the back, high up, but then we couldn't manage both situations where they scored.
"I thought we were good in the beginning but we need to maintain that level and that's what we are lacking all the time, that consistency over 90 minutes - or at least to keep a clean sheet for the first 45 minutes.
"That was our first target. We switched off, we expected there to be a foul called on someone but you can't switch off at any time in the game."
The Gunners have 10 days before their next match against Bayern Munich in the Champions League, and the Germany international believes his side need the time to mentally prepare for their clash against the Bundesliga leaders, who are leading 3-1 in the tie.
He added: "We need this breather now. We have 10 days until our next game so we can think about our situation.
"Of course there is potential but we need to show that on the pitch. That's what we need in the next few weeks, to go to Munich, to show responsibility for the Champions League," Mertesacker added.
"We have to take the chance there and afterwards we have to take care about the league and try to go for all points."