Iturraspe fouled Neymar just outside the box in the second half of the La Liga clash, but Martinez issued a yellow card, despite the 24-year-old appearing to be the last man.
"I think it would have changed the game," he told reporters at a press conference.
"He was bearing down on the goalkeeper. I don't know where [Athletic defender Mikel] Balenziaga was, but it was a clear one-on-one. To me it's a red card. The referee did not do his job well.
"We must continue on this line because we know it's going to be difficult."