Mikel Arteta was appointed as Arsenal manager in December 2019. His start to his first-ever managerial assignment was rewarded almost immediately, with an FA Cup win against Chelsea at the end of his debut season. Since then, Arteta has divided Arsenal fans with his approach.He may boast a handful of positives, but Arsenal endured their lowest league finish in 16 years and are set to be without European football for the first time in 24 years.As we approach an intriguing transfer window after his first full season at the club, let’s take a look at five mistakes Mikel Arteta has made at Arsenal.#5 Failed to identify his preferred defensive lineGabriel Magalhaes was signed from Lille for a reported 27 million poundsStarting off initially with a 3-5-2 formation, he preferred Bukayo Saka and Hector Bellerin as his wingbacks. Considering Saka’s offensive contributions, it was imperative to play him in a more advanced role. With Kieran Tierney’s return from injury and the subsequent signing of highly-rated Brazilian center-back Gabriel Magalhaes from Lille for a reported £27 million, it looked like Arteta would finally move over to the 4-4-2 he had envisioned since his appointment.However, 18 months later, it appears he is still unaware of his preferred formation and lineup at Arsenal. He has often been criticized for constantly switching his center-back partnership. Another common criticism has been the constant omission of Gabriel for David Luiz and others, thereby begging the question of why he was signed.Gabriel. "I feel, today, a much more complete player. Tactically, physically & technically, I feel extremely prepared. I played against the best players & at a very high intensity. This raised my level a lot" #Arsenal— Gurjit (@GurjitAFC) June 2, 2021Although Arteta deserves the liberty to experiment before he can figure things out for himself, the decision to leave Gabriel on the bench against Villarreal in arguably the biggest game of Arsenal’s season can only be described as baffling.#4 Failure to sign a backup left-back at ArsenalArsenal lack an understudy for Kieran Tierney in the left-back positionOne of the biggest mistakes that Arteta has committed is his negligence in signing a backup left-back. Kieran Tierney, as good as he has been for Arsenal this season, has a reputation of being injury-prone. In a system that relies heavily on wingbacks and crossing, not having a backup left-back in their ranks has cost Arsenal numerous times.Furthermore, taking into account the fact that Sead Kolasinac (Arsenal’s only other orthodox left-back) was loaned out in the same season, it appears Arteta has been unmindful of the impact on the team.In the past, players like Ainsley Maitland-Niles and Bukayo Saka have filled in for Tierney, but the pair don’t possess the attributes of an orthodox left-back. To add to this, Maitland-Niles was loaned out and Saka has since been converted to a right-winger.🗣 Kieran Tierney: "If you can see the differences in my game in the last 18 months or so it’s because of Mikel Arteta and what his coaching staff have been telling me to do."I love working with him, he’s helped me massively." [Scottish Sun] pic.twitter.com/DIAPv406No— Goal (@goal) June 1, 2021Cedric Soares has also been used as a left-back; he has exceeded expectations, but his single-footedness and lack of pace have exposed him in the past.These factors have forced Arteta to play Granit Xhaka at left-back. Although Xhaka is an experienced footballer who often pulls strings from midfield, the left-back position is not suited to his attributes, as he lacks the pace required to cover ground on the wing. Arteta likes to play with his wingbacks often overlapping the midfielders, something that we rarely see from Xhaka. His display against Villarreal’s Samuel Chukwueze was the final nail in the coffin; the culmination of Arteta’s negligence towards the left-back position.Also Read: Fabrizio Romano's Transfer Roundup: Update on Lionel Messi's Barcelona future, Chelsea's plan for the summer and more