The Gunners spurned a series of chances to take the lead against Watford in the FA Cup quarter-final at the Emirates Stadium on Sunday before Odion Ighalo opened the scoring after 50 minutes in an eventual 2-1 victory for the visitors.
That defeat leaves Arsene Wenger's side with only the slimmest of chances of silverware this season, with the North London club seemingly on the verge of a Champions League exit at the hands of Barcelona, and eight points behind Leicester City in the Premier League before the Foxes' clash with Newcastle United on Monday night.
Mertesacker insisted that the club's season is not all but over after their FA Cup exit, but admitted that they are not scoring as many goals as their style of football should be yielding.
The German centre-back told the Standard: "The season is not over. We play in a confident way, [but] we maybe lack that bit at the end. What is really important in football, to be successful we need to score.
"I think we had the chances and we played in the right way apart from the fact we didn’t score. That is something you need to do in football if you want to be successful. It looks like the longer the game goes, we rush and rush our decisions.
"It looks like we play, and play and play until the opposing team scores. That is something we need to work on. You have to give credit because we kept going and created chance after chance, but at the moment we lack that killer instinct. We need to work on that if we want to be successful in the Premier League and the Champions League."
Despite Mertesacker's insistence to the contrary, Arsenal's season has imploded in recent weeks due to a dismal run of form during which they have not taken their chances to win matches.
The Gunners have won just four of their last 14 matches in all competitions, winning just one of their last seven since Valentine's Day, the 4-0 FA Cup fifth-round replay win over Hull City last week.
Arsenal face a trip to the Nou Camp on Wednesday with the odds of staying in the Champions League stacked against them after Barcelona's 2-0 win at the Emirates last month.