Chelsea skipper John Terry said the signing of Cech would net Arsenal at least 12 extra points this season and, while Wenger refused to quantify his significance, he admitted his addition to the squad was key. “You cannot measure it,” he said when asked if Cech could prove to be worth 12-15 points. “You cannot measure that. But you know one thing: you need a top-class goalkeeper. I have said many times that it is an underrated position. And you need a top-class goalkeeper to do well.” Wenger has struggled for a commanding, first-choice goalkeeper since Jens Lehmann’s retirement but had two goalkeepers on his books before Cech’s arrival who he rates highly. Wojciech Szczesny has been farmed out on loan to Roma while David Ospina has had to settle for the job of understudy for much of the season – especially after a terrible error in Arsenal’s 3-2 Champions League defeat at home to Olympiacos. Wenger revealed he had to contemplate Cech’s signing because of how highly he regards Ospina but said he would have been wrong not to grasp the opportunity. “Look, it was a very, very difficult decision for me because I highly rate Ospina,” he added. “I have a high opinion of him. Ospina is a goalkeeper who, in my opinion, is underrated, especially in England. “But (signing Cech) was maybe one of the few opportunities to say, ‘okay, this is obvious’. It was more to say, ‘if I don’t do this, it would have been a mistake I think’.” Cech, 33, moved to Arsenal from Stamford Bridge in the summer having spent 11 hugely successful years with the Blues and Wenger now describes him as a “camera behind the team”. The Gunners boss made Cech’s signature a priority and the Czech Republic international was the only addition to his ranks before the start of the season. Since his arrival, Cech has claimed the record for Premier League clean sheets and has played a major role in guiding Arsenal to the top of the Barclays Premier League table. Meanwhile, Chelsea have endured a woeful defence of their title and sit 19 points behind their London rivals heading into their weekend clash at the Emirates Stadium and Wenger feels his defence has benefited from having such an experienced campaigner shoring them up. “He has a calming presence, he communicates well, anticipates well, sees what is coming,” he said. “He is a kind of coach. The best position to be a coach is the goalkeeper – because he doesn’t move, and he sees everything. He is the camera behind the team.” Wenger admits there were a few question marks over whether Cech would be up to the task of becoming Arsenal’s number one after a season warming the bench with the Blues but he insists any doubts were soon removed. “I didn’t know what kind of shape he was in physically because he hadn’t played,” he said. “When I met him I was quickly reassured of his desire to do well and his motivation. I didn’t know how much he could still produce physically. “He hadn’t played and was battling a drop in physical aptitude. But I knew the potential was exceptional having played against him 11 years in the Premier League. It was a question of ‘physically, can he still do it?'” Press Association Arsene Wenger will be hoping for a picture perfect performance from goalkeeper Petr Cech when he comes up against former club Chelsea on Sunday.