Arsenal defender Per Mertesacker has admitted that he has never played in a back three ahead of this afternoon's FA Cup final against Chelsea at Wembley.

The 32-year-old is expected to feature for just the second time all season in Arsenal's final game of the campaign, with manager Arsene Wenger facing a defensive crisis that sees Laurent Koscielny and Gabriel Paulista ruled out, while there is also a doubt over the fitness of Shkodran Mustafi.

Mertesacker's first appearance of the campaign came as a late substitute on the final day of the Premier League season last weekend, and the club captain admitted that he faces a steep learning curve if he is called upon for the final.

"I have never played in a back three, honestly. When I was young I started in a back four and that was my position from then on for the last 15 years. Everyone starts from zero with that new system," he told reporters.

"It means small adjustments but what it comes down to is knowing what the others are doing and giving them information that they know what you're doing. Communication is the key in that system, even more. We feel more compact even in the middle and accepting sometimes 'Just defend, just defend'. We're getting the ball sometimes so just allow ourselves to relax a little bit more.

"With a back four we felt we have to get the ball, we have to play, we have to press, especially against Manchester City (in the semi-final, which the Gunners won 2-1); we allowed ourselves just to defend from the box, not even open up too much.

"It's still a learning curve though for us, the back three, and for those who haven't played that system this season. We had to change to a back four against Everton due to that red card, but what made the difference against Everton was the communication, especially against each other. That's what we need to do on Saturday."

Victory for Arsenal at Wembley would see them lift a third FA Cup in the last four years and a record 13th overall.