Arsene Wenger revealed signing goalkeeper Petr Cech was one the hardest decisions he has had to make as Arsenal manager.
Cech joined the North Londoners after 11 years at Chelsea in a £10million deal this summer - but Wenger confessed he dithered and questioned the transfer before making the plunge.
The Frenchman says he was unsure over the fitness of the experienced Czech stopper and was worried about the mental affect it would have on the "underrated" former number one David Ospina.
"I didn't know what kind of shape he was in physically because he hadn't played. When I met him I was quickly reassured of his desire to do well and his motivation," Wenger said.
"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? It was a very, very difficult decision for me because I rate highly Ospina. I have a high opinion of him. Ospina is a keeper who, in my opinion, is underrated, especially in England.
"But [signing Cech] was maybe one of the few opportunities to say, 'OK, this is an obvious [transfer]. If I don't do this, it would have been a mistake I think'."
Following a nervy debut performance at home to West Ham, Cech has settled seamlessly into Arsenal first-team, forming a more resolute back-line, which has conceded just 21 goals in an unpredictable Premier League campaign.
Wenger acknowledged the importance of having a world-class goalkeeper and believes Cech inspires belief throughout his team.
"He has a calming presence, he communicates well, anticipates well, sees what is coming. He is a kind of coach," Wenger said.
"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.
"You cannot measure it. 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."