Petr Cech has been dropped by Arsène Wenger for the FA Cup final against Chelsea on Saturday and David Ospina will make a surprise start in goal. Cech is devastated by the decision. It was thought that he suffered a slight knock in training but he is now ready to take his place on the bench.

Wenger has had to mix and match at the back in the face of selection problems and he will ask Alex Oxlade-Chamberlain, who is fit again after hamstring trouble, to step in at left wing-back in his 3-4-2-1 formation. Hector Bellerín will continue at right wing-back. The left-back, Kieran Gibbs, has been struggling with a thigh problem.

Wenger’s major doubt concerns Shkodran Mustafi, who suffered a concussion in the Premier League win at home to Sunderland, and the centre-half is not expected to be fit for Wembley.

Two other centre-backs – Laurent Koscielny and Gabriel Paulista – are out through suspension and knee ligament damage respectively, so Wenger is set to turn to Per Mertesacker in the position. Mertesacker returned to action as a substitute in the home win over Everton on Sunday after nearly 13 months out with a knee injury.

Mertesacker said he has “never played in a back three”. He added: “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.

“Am I able to play for 90 minutes? I cannot tell you. I haven’t played this year. But I have done this for 15 years, so I expect myself to be absolutely ready, no matter what comes. I am going to go for it. I owe it to this team and this club. I want to make sure I will be ready and my mind-set is 120 minutes, plus penalties.”

Wenger had suggested on Wednesday that he would start with Cech over Ospina, despite having favoured his back-up goalkeeper on FA Cup runs. Ospina played in all eight of Arsenal’s Champions League ties this season, plus the first three rounds of the FA Cup.

Wenger stressed that Cech, his No1, had started in the semi-final win over Manchester City last month. “Will Cech play against Chelsea? It is possible,” Wenger said. “We will see.” Ospina, who is expected to leave the club in the summer, now has his chance.