The Chile international has enjoyed a superb season for an Arsenal side that has again struggled to sustain a credible title bid.
Sanchez has scored 21 Premier League goals and recorded 10 assists but his future remains a matter of intense speculation.
The 28-year-old’s contract expires in just over a year’s time and he has been linked with potential moves to Chelsea and Bayern Munich.
Koscielny, who himself extended his contract earlier this season, acknowledges what losing a player of Sanchez’s standing would do to morale and spirit.
‘Firstly, he [Sanchez] scored a lot of goals for us but his first job is not to be a striker, it is more midfield but he is an unbelievable player. He just wants to fight and have the best result for the club, he told Sky Sports.
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‘Sometimes he is disappointed with the result of our game but he knows that we just want to fight for each other and for the club. When you have a player like this in your team, to show the way we want to grow together, it is important and I think he has a lot of players behind him to keep him.
‘I don’t think there are a lot of players like him and a player like this who can be the difference during the game, it’s rare. We want to keep him with us because we know he is so important for our team.’