The Frenchman and fellow substitute Aaron Ramsay scored in the final seven minutes as Arsenal twice came from behind, the Welshman levelling two minutes before Giroud headed the decider.
With the likes of Laurent Koscielny and Shkodran Mustafi still to come back, Wenger will no doubt be hoping his side will tighten up at the back as the season progresses, while the return of Alexis Sanchez (if he stays) will obviously strengthen the Gunners' attack.
Giroud then sent the previously despondent Arsenal fans wild when he headed the 85th minute victor from a corner, Leicester keeper Kasper Schmeichel only managing to turn the ball onto the underside of the bar before it bounced over the line.
Vardy's second goal, a towering header from a corner-kick, looked to have won it for the Foxes until the sensational finale that saw the game swing in Arsenal's favour.
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"Not only as a player, he is a fantastic player, b ut as well in the group - he has a big weight in the squad and it was a relief for me that he made a decision to stay".
Wenger has always been desperate to keep Giroud because he offers something so different with his aerial presence but was also willing to let him leave if he felt that he needed a guarantee that he would start more regularly in this World Cup season.
"You come away from home, score three goals and are almost there and then concede two.bitterly disappointed", he said.
"We were quite good previous year on corners and I believe with a bit of work, we can get that out of the system".
"I think we have all seen the amusing business of the transfer window and we never know what is round the corner". I thought the ball would go out as it was flying over me and suddenly there was a player.