The two sides meet again following last year's third-round tie, which forced a replay at Elland Road.
"It is the same in Germany [as in England]," Mertesacker told the club's official website. "The leagues are very strong and [teams in] the second or third leagues are also professionals who train a lot of time in the week, just like in the Premier League or Bundesliga.
"The difference is the speed. [Lower-league teams] can fight, run and play good football, but you have to be very confident from the first moment [against them]. Confident that you are part of the Premier League and the Champions League.
"The FA Cup in Germany is very famous and it is very good to be part of it. It is a great feeling. Hopefully we will go through the next few rounds."
The Gunners will face fellow Premier League side Aston Villa at the Emirates if they defeat Leeds this evening.