Barcelona dropped more points in La Liga after failing to beat Alaves on Saturday as their first game since the sacking of Ronald Koeman ended in a disappointing 1-1 draw at Camp Nou.
Sergio Aguero had to be taken to hospital for tests after suffering breathing problems during the first half of the game.
Aguero was able to walk off the pitch and Barca's interim manager Sergi Barjuan said afterwards: "I asked him and he told me he was dizzy. I was told he went to hospital. I don't know anything else."
Gerard Pique was also taken off with a calf strain and, along with Aguero, will now be a doubt for Tuesday's crunch Champions League game away at Dynamo Kiev.
Xavi Hernandez is expected to be appointed Koeman's successor this week but with the details of his move still to be agreed with Qatari side Al Sadd, Barca B coach Barjuan has been put in temporary charge.
Yet the removal of Koeman, and even the prospect of Xavi, produced no obvious uplift as a superb second-half strike from Memphis Depay was quickly cancelled out by an equally impressive run and finish from Alaves' Luis Rioja.
"What I have to do is win over the players, make them believe in themselves," said Barjuan.
After three league games without a win, Barcelona stay ninth, eight points adrift of leaders Real Madrid, who earlier defeated Elche 2-1 thanks to two goals from Vinicius Junior and aided by a red card for Elche's Raul Guti.
Even more worrying for Barca will be the gap behind the top four is five points while attendances suggest the team's form is creating a sense of apathy amongst supporters, with only 37,238 inside the 99,000 Camp Nou.
Club president Joan Laporta has suggested Barjuan will still be in charge for the trip to Kiev, with Xavi, or the next coach, arriving in time for next weekend's game against Celta Vigo or during the international break.
Barjuan admitted on Friday that it is "difficult to change things in a short time" but also said the team's style, a key factor in Koeman's dismissal, is "non-negotiable".
But while the first half an hour was slightly more purposeful from Barcelona, they still struggled for chances and the defence was beaten too easily for Rioja's equaliser.
For all the talk about style and possession, Depay's opener was a thunderbolt out of the blue. Jordi Alba played inside and Depay opened up his body before curling the ball into the top right-hand corner.
But Barca's lead lasted less than three minutes as Alaves equalised with another superb goal, Rioja dancing past three Barca defenders, latching onto a sublime flick from Joselu, before rounding Marc-Andre ter Stegen and finishing.
Depay was slipped through by Gavi and Nico Gonzalez but hit the post and then Antonio Sivera as Alaves held on for a hard-fought point.
Earlier, Vinicius' sensational start to the season continued as he scored twice in Madrid's victory over Elche to ensure the rested Karim Benzema was not missed.
Vinicius had already given Madrid the lead when Elche's Guti was sent off in the 63rd minute and he added a second soon after with a superb chipped finish into the corner.
Madrid made life difficult for themselves late on when a Casemiro mistake allowed Pere Milla to pull a goal back but Elche's brief hope of a comeback came to nothing.
"We were very practical," said Madrid coach Carlo Ancelotti. "After only two days rest it's not possible to play with intensity and energy."
Benzema's resurgence for France means Madrid no longer have their chief goalscorer all to themselves and after 17 games played already this term, Ancelotti opted to leave the striker at home.
It meant a surprise start for Mariano Diaz but it was Vinicius who filled the void, scoring his eighth and ninth goals of the season to continue an excellent run of form.
The 21-year-old Brazilian has long been circled as one of Madrid's most exciting talents but he has found a different level under Ancelotti.
"I haven't done anything," Ancelotti said. "I just try to give him confidence. I am not a magician."