Radamel Falcao made a goalscoring return for Monaco, who beat Arsenal 1-0 in the Emirates Cup.
Having battled back from a serious knee injury, the forward scored a first-half header to frustrate Arsenal in their own tournament.
The result denied Arsenal victory in the Emirates Cup. Instead, Valencia, who beat Benfica earlier in the day, took home the ultimate prize.
Monaco provided far stiffer opposition than Benfica did 24 hours previous and a cross-shot from Nabil Dirar almost embarrassed Wojciech Szczesny. The ball beat the keeper, but it drifted across the face of goal.
Arsenal’s midfield looked extremely vibrant, and some slick passing created space for Aaron Ramsey, but he went for a pass when a shot was on, and the chance went begging.
On 19 minutes, Falcao served notice of his aerial prowess when climbing above Calum Chambers and sending a header narrowly wide.
If that was a warning for Arsenal, it was one they did not heed, as the Colombian fired his side ahead on 37 minutes.
Jack Wilshere was penalised for lunging in on Dirar, and it proved a costly foul to give away.
The excellent Joao Moutinho delivered an inviting ball into the box. It was an invitation Falcao took up, as he rose high to head past Szczesny.
It was Falcao’s first goal since suffering a serious knee injury in January, but it came gift-wrapped from the Arsenal defence as Laurent Koscielny inexplicably elected to throw his hand in the air and claim offside rather than track the run of the forward.
Monaco could have doubled their lead moments before the break as Dirar was not tracked at a corner, but his audacious flick flew just wide of the post.
Arsenal threw on Alex Oxlade-Chamberlain at the break, but it was Monaco who went close at the start of the second half. The Gunners gave the ball away in sloppy fashion, and Elderson Echiejile fired a shot just over the top.
Arsenal were denied a leveller on 50 minutes by a brilliant save from Danijel Subasic. A deft back heel from Alexis Sanchez set Wilshere free. The England midfielder lashed his shot on target, but Subasic threw out a hand to keep the ball out.
Falcao’s involvement in the game ended shortly before the hour mark, as he was replaced by coach Leonardo Jardim, but it was an excellent hour’s work for a forward making his way back from a serious knee injury.
The introduction of Oxlade-Chamberlain added some spark to Arsenal, and his dynamic play caused problems for the Monaco defence.
Arsenal appeared to be running out of ideas when a huge error was made that cost them a chance of claiming a draw.
Chuba Akpom burst clear and was felled in the box by Subasic. Referee Martin Atkinson pointed to the spot, but then he changed his mind and awarded a free-kick.
Replays showed that the challenge was well inside the box and that Atkinson had made a mistake.
The Arsenal protests would arguably have been more vehement had it been a Premier League or Champions League match, but it still denied them the chance to claim the point that would have secured them the Emirates Cup.
But they did not, and it allowed Valencia to take the trophy.
Arsene Wenger spoke to BT Sport after the game:
It is a pre-season friendly so we don’t want to start moaning about the referee.
I don’t think we were on the level physically. Monaco were sharper, stronger and won the challenges. If you do not win the battles in the midfield you don’t win the game.
Ideally you want to win every game, but if you choose to lose one it is a pre-season game.
It is a warning as it shows we are not yet there physically.
Arsenal's focus will now shift to the Community Shield, which will see them take on Manchester City next weekend. Following that, the season proper gets underway—with the Gunners taking on Crystal Palace at home on August 16.
There were positives to take over the course of the two days, with Yaya Sanogo and Joel Campbell impressing against Benfica, but defensive frailties were on show against Monaco. With Thomas Vermaelen possibly heading for the exit door, Wenger may well see the need to bring in a centre-half.