Lionel Messi's 30th La Liga goal of the campaign was overturned by a Santi Mina brace on Sunday evening as Celta Vigo shocked Barcelona to win 2-1 at Camp Nou, ending La Blaugrana's title charge.
Barça dominated possession from the get-go and managed to make it count after 28 minutes. Sergio Busquets delivered an inch-perfect, floated cross onto the head of Messi, who directed the ball past Ivan Villar without breaking stride.
Koeman's men found themselves pegged back just ten minutes later, however, when Mina's strike from outside the area wrongfooted Marc-Andre ter Stegen to make it 1-1 at the break.
Barça looked the more likely to pull ahead in the second half with Ronald Araujo going closest, heading narrowly over the bar from a set piece. However, things were made difficult in the 83rd minute when Clement Lenglet earned himself a second yellow card for a rash challenge on Kevin Vazquez.
La Blaugrana pushed for a winning goal but the visitors managed to find one in the 89th minute. Kevin Vazquez's overcooked cross crashed against the post and came out perfectly for Mina to thrash into the back of the net, stealing the three points at the death.
The loss leaves Barcelona third in the table and now seven points off leaders Atletico - who simultaneously registered a 2-1 win - with one match remaining. So, in a heart-breaking game, here are you Barça player ratings.
BARCELONA PLAYER RATINGS
1. Goalkeeper & defenders
Marc-Andre ter Stegen (GK) - 5/10 - Will be annoyed to have conceded Celta's opener and was desperately unlucky with the second.
Ronald Araujo (CB) - 6/10 - Kept things simple on the ball and positioned himself well off it, effectively covering the space in behind Ousmane Dembele - and then Sergino Dest - constantly.
Gerard Pique (CB) - 6/10 - Was strong for the most part and usually did enough to deny the visitors, notably producing some excellent last ditch defending early on. However, he was caught out for Celta's equaliser, giving Mina far too much room to shoot.
Clement Lenglet (CB) - 3/10 - Pressed high and distributed the ball nicely but got himself sent off with a second booking - silly and costly.
2. Wing backs & midfielders
Ousmane Dembele (RWB) - 7/10 - Was positive and dangerous throughout, running powerfully and directly at the opposition back line. His pace imbalanced and cut through Celta brilliantly, but the Frenchman failed to punish them.
Ilaix Moriba (CM) - 6/10 - Was strong and combative against his opposite numbers, showcasing his athleticism but failed to do anything of serious note and was too passive in possession.
Sergio Busquets (CM) - 7/10 - Really enjoyed himself on the ball, making Barça tick with his effective recycling of possession. Produced one of the moments of the game with a splendid ping onto Messi's forehead, but was once again sluggish in dealing with the counter.
Pedri (CM) - 6/10 - Floated around and linked up nicely with the two ahead of him. Couldn't quite make an impact where it counted and was harshly hooked off at half time.
Jordi Alba (LWB) - 6/10 - Picked out some lovely positions up the left flank and managed to get in behind on a couple of occasions, but failed to produce quality when delivering into the area.
Lionel Messi (ST) - 8/10 - As usual dropped off and always found pockets of space in which he could link up cleverly and precisely with team mates, influencing proceedings throughout. Was wasteful in front of goal but did finish well to grab the opener. Couldn't inspire his side to victory this time.
Antoine Griezmann (ST) - 7/10 - Buzzed around constantly and was a thorn in the side of Celta's defence. Showed great movement to create openings for others and managed three key passes, mostly combining with Messi to great effect.
Riqui Puig (CM) - 6/10 - Added dynamism, directness and drive form midfield, often surging forward and creating opportunities, but couldn't come up with a decisive moment.
Sergino Dest (RB) - 5/10
Martin Braithwaite (ST) - 5/10
Francisco Trincao (RW) - 5/10
Miralem Pjanic (CM) - N/A