La Blaugrana looked on course for an eighth successive league win against Alvaro Cervera’s side, after Lionel Messi capped his record breaking 506th club appearance with a first half penalty.
However, an 89th minute penalty from Alex Fernandez snatched a dramatic point from the visitors, as Barcelona lost vital ground in the title race.
Koeman was at the final whistle, and he claimed the dropped points were more of a blow than their 4-1 Champions League humbling by Paris Saint-Germain in midweek.
“I am very very disappointed, more than I was last Tuesday (against PSG),” he told a post match interview with Marca.
The draw means Barcelona have missed the chance to overtake Real Madrid and return to second place in the table, with a new five-point gap behind Los Blancos, albeit with a game in hand.