Arsenal are yet to seal their spot in the last 16 of the Champions League after they slumped to a 2-0 defeat at Sporting Braga on Tuesday.
Arsene Wenger will be furious after his side were denied what appeared to be a clear penalty before conceding two late goals in Portugal and falling to their second consecutive defeat in the Champions League.
In more bad news for Wenger, captain Cesc Fabregas came off with a hamstring injury and is in doubt for Saturday's trip to Aston Villa in the Premier League.
Arsenal served up a 6-0 hammering to Braga when the two sides met last time out in London, but the Gunners could not capitalise on having the bulk of possession at Estadio Municipal.
Arsenal needed only a point to ensure their place in the last 16, and for most of the match a 0-0 draw appeared the most likely result.
Theo Walcott went close after a good through ball from Fabregas, but Braga goalkeeper Felipe was quick off his line to deal with the danger.
Fabregas was withdrawn soon afterwards, with replacement Samir Nasri lively upon his introduction.
The turning point of the match came in the 77th minute when Alberto Rodriguez appeared to bring Carlos Vela down inside the box after Nasri had sent the Mexican striker clear.
But instead of awarding a penalty, Hungarian referee Viktor Kassai gave Vela a yellow card for diving, much to the disgust of the Arsenal bench.
And barely six minutes later Braga were in front, with Matheus racing onto a loose ball from deep in his own half before slotting the ball past goalkeeper Lukasz Fabianski.
Matheus settled the matter deep into stoppage time, surging into the box and holding off three defenders before slamming a left footed shot off the frame of the goal and into the net.
The loss leaves Arsenal second in the Group H standings on nine points, even with Braga and three behind leaders Shakhtar Donetsk, who won 3-0 against Partizan Belgrade.
Arsenal can still qualify for the knockout stages with a win at home against minnows Partizan in a fortnight, while a draw could also see them through depending on the result of the match between Braga and Shakhtar.