Bellingham was sent off by referee Jose Luis Munuera Montero for alleged foul and abusive language during an Osasuna match.
The referee claimed the 21-year-old shouted “f*** you,” while Bellingham maintains he said “f*** off” in frustration.
"My English is good, but I don't understand the difference between f*** off and f*** you," said Guardiola.
"But what's important is the intention, not the insult. The best thing is not to do it and leave the referees in peace.
"There have always been decisions. You should ask Jude what his intention was. That's what matters.
"Maybe you can insult someone with a great smile on your face and that wouldn't be that bad, right? I read that there is an investigation - wow. Come on. Leave him (the referee) alone."