Vermaelen, who left the Londoners for the Catalan giants in 2014, said his old club must still be regarded as a serious threat even though they lost the first leg 2-0 at the Emirates Stadium last month.
"Of course we will be the favourites but I know Arsenal and I know it's not going to be easy," he said on the eve of the match. "You can never say they are out because they will always come back.
"They have pace and are physical. They have very quick players on the wing and are very energetic, that's what makes them dangerous," the Belgium international told the Arsenal website (www.arsenal.com).
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Manager Arsene Wenger, who has come under fire following a dismal run of one win in seven matches in all competitions, said it would be important for his Arsenal side to be solid at the back.
"There are no magic words to win the game," the Frenchman added. "I think the key for us will be having a good defensive line.
"We will attack knowing that to counter the magicians of Barcelona we need a good defensive performance."
Wenger dismissed the criticism he has received from some fans following Arsenal's FA Cup quarter-final elimination at the hands of Watford and their slide to third in the Premier League table, 11 points behind leaders Leicester City.
"I only want to give my best for this club, that's mydedication and my commitment," he said.
"We have gone through a disappointing spell and it is important to remain calm and strong."
(Writing by Amadeu Garcia; Editing by Tony Jimenez)
