Slaven Bilic's men impressed against the Spaniards, frustrating them with dogged defending and creating numerous chances, only to lose out to Jesus Navas' winner in the closing minutes.
Croatia had two separate shouts for penalties turned down by referee Wolfgang Stark, decisions that Srna says would have been given in Spain's favor.
The 30-year-old also expressed his belief that La Roja will need new ideas if they are to go on and win the tournament.
"We showed other teams how to play against Spain - but we forgot about the referee," the defender is quoted in the Daily Star. "Spain have to change the way they play if they want to win the Euros. They found it difficult against Italy and us. We played better than Spain but the referee refused to give us two penalties. It's a very difficult moment for us.
"Referees don't respect us, although Croatia in the last six years has been among the best 10 teams in Europe. If they had the same situations for Spain, they would give a penalty. This is a problem for the smaller nations when they play Spain."
The Shakhtar Donetsk man also sarcastically suggest that for other teams to defeat the Group C winner, they will need a different official.
"I don't know what the other teams will have to do to beat them [Spain]," he said. "Maybe they will have to change the referee."
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