The Germany international says his team must be more ruthless in front of goal if they are to be successful and says they simply cannot afford to concede against Barcelona
The Gunners were knocked out of the FA Cup on Sunday after slipping to a 2-1 defeat to Watford with Danny Welbeck guilty of missing an open goal in the final minutes at the Emirates.
The cup exit follows defeats to Swansea, Manchester United and Barcelona and Mertesacker says his team need to improve their attack.
"What we do lack at the moment is we need to score at some point. It looks like we play, and play and play until the opposing team scores," Mertesacker told reporters.
"That is something we need to work on. We had the opportunities and the possibilities when they [Watford] were 2-0 up to come back and we kept going.
"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.
"It looks like the longer the game goes, we rush and rush our decisions. It is still a learning process for us as a team."
Next up for the Gunners is the second leg of their Champions League clash with Barcelona at Camp Nou and Mertesacker feels they cannot afford to concede against the holders.
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"The season is not over. We play in a confident way, we maybe lack that bit at the end. What is really important in football, to be successful we need to score," added Mertesacker.
"We cannot afford to concede [against Barcelona]. We have to go there and try to attack, no matter what. That is the only possibility we have.
"We can be fortunate to have such a big game in front of us. I cannot predict [the result], it is really hard. It is the ultimate team at the minute, so we are straight away under pressure and we have to go away from home."