#2 Olivier Giroud
Olivier Giroud was another player who left Arsenal in January 2018. The Frenchman left the North London club in search of more consistent game time so that he would not lose his spot in Didier Deschamps' World Cup squad. Alexandre Lacazette had signed for the Gunners that season, and started to replace Giroud in the starting eleven, and he shifted to join Chelsea in a deal worth £18 million.
As much as the centre-forward was proficient in scoring headed goals, he did not have the attributes of a top-class striker that Arsenal fans had envisioned. The fans wanted someone like Robin van Persie who could run fast and was capable of scoring goals by himself. Unfortunately, Giroud was not that kind of player.
When Chelsea and Arsenal met in the Europa League final, Giroud was a decisive player in the Blues' victory. His opener affected Arsenal's confidence in the game and helped his side to a comfortable victory.
Arsenal lost Danny Welbeck in November 2018 and since then, they have been reliant solely on Pierre-Emerick Aubameyang and Alexandre Lacazette to score goals. If the 32-year-old did not leave Emirates, he would have provided another attacking option on the bench which could have bolstered Arsenal's attack. Letting Giroud leave the club was surely one of the worst decisions in 2018.