Giroud was ruled out for up to four months after suffering a broken tibia in the 2-2 Premier League draw with Everton last month.
Koscielny said his France team-mate will be sorely missed by the London club, but expects the former Montpellier man to make a big impact when he regains full fitness.
The defender told the club's official website: "Olivier is an important player for us.
"He's very strong during the game and he's scored a lot of goals. So for us it's very difficult to lose him. But I know him and I know he will come back stronger.
"It is important for us that Olivier keeps this mentality and comes back stronger for us.
"He will want to show everyone that he can be a very good player and a good striker in the Premier League and in Europe. It is important for us that Olivier keeps this mentality and comes back stronger for us.
"We have a lot of players in the forward positions now so we have a solution. We played with Alexis [Sanchez], Yaya Sanogo and Lukas Podolski, so we have a solution and now we need to be a unit and to prepare the same way to win the game.
"When you are injured for three, four or five months, the most difficult thing is to keep motivated and be happy enough to come back. Some days you could just say I don’t want to come back - I can't arrive this week. We need to be with him.
"He knows all of the players are with him, and we are behind him to help him to come back quickly, stronger, because we need to play with him to score a lot of goals. I think he will be very important for us in the last six months of the season.
"For a striker we talk a lot about his goals, but when you watch Olivier during a game you can see his job, which is not just offensively but also defensively because he can come back for free-kicks to help us.
"When we play the long ball, he can keep the ball and he’s strong so the defenders can't touch the ball. He's very, very important for us. Now we need to do without him and I hope there is a player who can replace him and do the job well."
