Sanchez scored twice as the Gunners beat West Brom to go back above Manchester City and put some distance between themselves and Manchester United.
But he started the season slowly after helping Chile to win last summer’s Copa America, and then missed two months of the campaign through injury.
Looking back, Mertesacker admits that is one of the reasons why Arsenal are now clinging on to a Champions League spot instead of fighting for the title.
He said: “There are a couple of things we missed but this is obviously one.
“When he was out, I think the statistics were not that bad for us – I think we played ten games without him and won eight or something like that.
“But obviously if he is on the pitch, he creates space always for other players because he draws so much attention and that is the key to any team.
“He is a key member of our team and if he is fully fit, he makes the difference and that is always the case.