Arsenal lost ground in the race to finish in the Premier League's top four after losing 1-0 to Sheffield United at Bramall Lane on Monday night.
Both teams laboured through some fairly torpid opening stages, with the hosts content to go long, while Arsenal struggled to assert their passing game on the deck.
Going direct seemed like a better route, and Arsenal had a penalty claim waved away by Mike Dean, despite Sokratis Papastathopoulos having his shirt pulled in the box:
The Gunners appeared to be lacking ideas but should still have gone in front on 21 minutes when a slick counter started by Nicolas Pepe ended with the winger scuffing his finish from close range after a fine cross from Sead Kolasinac on the overlap:
It was the kind of missed opportunity indicative of a player lacking confidence amid a tough time adapting to a new league.
Arsenal's collective confidence was shaken when Mousset added to the cost of Pepe's miss on the half-hour mark. The striker reacted quickest in the box to prod in after Jack O'Connell nodded down from a corner.
Mousset, signed from Bournemouth in the summer hasn't been a regular in starting lineups, but he has still managed to make a habit of punishing the Gunners:
Familiarity aside, the Blades' goal was also the result of an inherent weakness in this Arsenal team since Unai Emery replaced Arsene Wenger in the dugout in 2018:
Arsenal's response saw a penalty appeal denied and Bukayo Saka booked when the 18-year-old winger went down amid a tussle with George Baldock in the area. Gunners coach Freddie Ljungberg was booked for his response to referee Mike Dean's decision:
The visitors couldn't muster the same intensity as Ljungberg and continued to toil in a pedestrian manner going forward. Granit Xhaka ended the half forcing Blades goalkeeper Dean Henderson into a save for the cameras to end a limp 45 minutes from Arsenal.
Needing more craft in possession, Emery hooked Joe Willock for Real Madrid loanee Dani Ceballos at the break, and the Spaniard soon became an active presence between the midfield and forward lines. Yet for all his endeavour, Ceballos couldn't find his range.
The Spaniard's struggles had some questioning the ongoing absence of Mesut Ozil, Arsenal's highest-earner and the lone natural No. 10 in the squad who has so far failed to convince Emery of his worth.
Ozil's absence showed up in Arsenal's inability to make the most of owning the ball. The Blades were obviously content to cede possession and wait to strike on the counter, but the Gunners didn't have the ability to punish them for resting on such a slender lead.
Emery tried to force more impetus by replacing Xhaka with centre-forward Alexandre Lacazette, freshly returned from injury. It hardly mattered though, with Pepe giving Henderson a slight sting on the palms from a free-kick being Arsenal's closest thing to a positive moment going forward.
The winger flashed an effort narrowly wide just after, but too few of his team-mates were matching Pepe's application and enterprise. It meant a tame finish to a drab performance from a team with top-four ambitions but lacking the style that once made that place close to a guarantee.
By contrast, the Blades merit genuine praise for the way they have played to their strengths and shown little fear against the division's big boys.
Arsenal are on UEFA Europa League duty against Vitoria in Group F on Thursday, while the Blades return to domestic action for a game away to West Ham United on Saturday.