Chris Wilder was at a loss to understand why VAR didn't intervene when David Luiz clearly pulled back Oli Burke in the first half of Sunday's clash at the Emirates Stadium.
In a similar incident to the one that saw John Egan dismissed at Villa Park last month, referee Lee Mason didn't even award the Blades a free kick, much to the bemusement and annoyance of the Blades boss.
United were comfortable in the first period in the capital and Wilder praised his side for their stubborn and resilient approach, only to see goals from Bukayo Saka and Nicolas Pepe swing the game decisively in the Gunners favour just after the hour mark.
Characteristically, the Blades refused to throw in the towel though and David McGoldrick's late effort gave the visitors hope of claiming an unlikely point which impressed the Blades manager.
Meanwhile, Wilder confirmed he is in the market for defensive reinforcements before Monday's transfer deadline, insisting any deal will be for the benefit of the squad and most importantly the football club.