Charlie Nicholas has warned Arsenal that they have to improve defensively if they are to be considered realistic challengers for the Premier League title.

The Gunners fell to a 2-0 defeat in their season opener as keeper Petr Cech committed two errors to allow Cheikhou Kouyate and Mauro Zarate fire Slaven Bilic's men to victory at the Emirates Stadium.

Nicholas believes that Arsene Wenger's men were over confident heading into the season, but feels that the shock of the loss will jolt the club into action.

He told Sky Sports News: "They we're too cocky and paid the price. I'll take nothing away from West Ham who were excellent but it has been an Achilles heel of theirs for a long time.

"It was one heck of a reminder to say that you can have all the good ingredients going forward but if you don't sort your defence out you won't win anything.

"It was an eye opener, a wake-up call, but I'm glad, if anything, it happened on the first weekend of the season rather than well into the season."

Arsenal play Crystal Palace in their next match at Selhurst Park on Sunday.