#3 Lack of support from Arsenal's board

Arsenal, being 7th on the list of most valuable football clubs across the globe, had generated expectations in their fanbase that the board will cover up for Unai Emery by signing a defender as a temporary replacement for Holding, a winger to replace Mkhitaryan and a holding midfielder to give the likes of Torreira, Guendouzi and Xhaka some rest.
But instead, Unai Emery reportedly claimed that Arsenal could only do loan deals in the winter transfer market, as they had no funds due to various reasons. This left the fans annoyed. Well, they could do nothing but to accept the fate.
Arsenal only made one signing and it was Unai Emery's former player Denis Suarez; a Barcelona outcast and former Manchester City player. Despite the injury issues in the defence, the board had no plans whatsoever to bring in defenders, leaving it to Unai Emery to work with what he had at his disposal.