Ernesto Valverde called the penalty decision that cost Barcelona victory at Las Palmas "invisible" and admitted he was still none the wiser over why it was given.
Unbeaten La Liga leaders Barca dropped points for the third time in five league games as they were held to a 1-1 draw at struggling Las Palmas.
Valverde's side led through Lionel Messi's free-kick but were pegged back after the break by Jonathan Calleri's spot-kick, which was seemingly awarded for what was deemed a deliberate handball against Lucas Digne, rather than a tug on Matias Aguirregaray.
Both would have seemed harsh and Valverde was furious with referee Antonio Mateu, particularly after goalkeeper Leandro Chichizola had gone unpunished on the stroke of half-time when he appeared to deliberately handle after rushing out of his box.
"We don't know what the referee blew for during that play," Valverde said in his post-match press conference in quotes published on AS.com.
"Nobody knows in the dressing room. In that phase of play we don't understand what he blew for, it was inexplicable. We hope that his decision will become a little clearer when we see a replay. It was an invisible penalty."
On Chichizola's handball, Valverde added: "For me, it was really clear."
Though Barcelona are yet to lose in their domestic division, six of the 12 points they have dropped all term have come in their past five contests.
That form has given second-placed Atletico Madrid hope they can overhaul Barcelona, and they visit the Nou Camp this weekend having won eight in a row in all competitions.
"The match against Atletico comes up fast and it'll be important for both teams," Valverde added.
"We go into it with a bad taste in our mouths, because we could have got a win. We're the leaders and we're keen to stay in first place."
Watch Barcelona v Atletico Madrid live on Sky Sports Mix from 3pm on Sunday.