Arsenal manager Mikel Arteta has made a startling admission regarding his tactical preparation after his side narrowly avoided an FA Cup upset against Mansfield Town. The Premier League leaders were forced to dig deep to secure a 2-1 victory over the League One outfit, but the Spaniard revealed after the final whistle that his experimental starting XI was barely tested on the training ground before the trip to Field Mill.The Gunners lined up with a makeshift three-man defence featuring teenager Marli Salmon, Cristhian Mosquera, and Riccardo Calafiori. Noni Madueke and Gabriel Martinelli provided width as wingers, rather than as more traditional wing-backs, meaning the game became stretched with fewer bodies back for Arsenal.The decision to pivot to a back three was born out of necessity rather than design, with several key first-team regulars either sidelined or in need of a rest. William Saliba remains out through injury, while Gabriel Magalhaes, Declan Rice, and Martin Zubimendi were all left out of the starting XI to manage their workloads during a gruelling period of the season.When questioned about how much time the squad had spent working on the unconventional setup, Arteta offered a blunt response that highlighted the chaotic nature of their preparation.Asked how many training sessions he had practised this set-up in training, Arteta said: "Yes, once. Ten minutes."Addressing the selection headache, Arteta explained the logic behind omitting his star names despite the risks involved in the cup tie. Arteta said: "Well, they had issues and they’ve been carrying issues. And the amount of games that we are playing, it’s obviously super-demanding. And it was the only time to try to make sure that we can clean those issues that they’re having to start the next sequence of two weeks before the break in the best possible way. And we had to make those decisions. It was a challenge for all of us to see how we could adapt to that, which I expected in very difficult circumstances."While Arsenal remain alive in the FA Cup, the victory came at a potential cost. Both Leandro Trossard and Riccardo Calafiori were withdrawn during the match, adding to the growing list of fitness concerns at the Emirates Stadium. The manager confirmed that he was forced into the changes to prevent long-term damage during the physical encounter.Speaking on the condition of the pair, Arteta said: "Both of them had little niggles. They were not comfortable to continue. And I knew that this could be a possibility, especially in the conditions that we’re playing today. So we had to take them off."Arsenal will now be sweating on the availability of their key stars as the business end of the season approaches.Add GOAL.com as a preferred source on Google to see more of our reportingThe narrow escape in the FA Cup serves as a wake-up call for the north London side, who have relied heavily on defensive solidity throughout their domestic campaign. The Gunners prepare for their next sequence of fixtures before the international break. Arsenal travel to the BayArena to face Bayer Leverkusen in the Champions League last 16 on Wednesday night before welcoming Everton in the Premier League three days later.