By mid-September Arsenal were still in 17th place but they have staged a remarkable turnaround and will finish third if they win at West Brom on Sunday.
Koscielny told France Football: "The start of the season was catastrophic. We lost key players from our system. There was late recruitment of new players and the mayonnaise needed time to 'take'.
"There was the thrashing at Old Trafford. The whole world buried us.
"But you don't lose your football like that.
"It can happen that a small grain of sand jams the engine and after that you start to doubt your own qualities, the club, the coach and everything else. We needed to sort that out. Everyone got down to doing just that, the coach, the club."