Having been chosen to officiate the showpiece fixture, Webb showed the former Manchester City midfielder a yellow card after a recklessly high tackle on Alonso in the 28th minute of the fixture, which ended 1-0 after Andres Iniesta's extra-time strike.
Webb was subsequently booed when he collected his medal after the final whistle, and he has now conceded that he made a mistake in refusing to dismiss the Dutchman, but has suggested that he did not see the incident clearly enough to take such a decision.
Speaking to BT Sport, Webb said: “I still thought I got it right on the pitch. So I get back into the dressing room and my assistant referee has gone to his pocket and got his phone out, and his face dropped.
“I said ‘what’s wrong?’ and he said ‘De Jong should have been red carded, the guy from the referee headquarters saying he should have been red carded’.
“I went ‘you’re joking?’. I got my phone out, a mate of mine in the police said ‘Howard, it’s not a red card, it’s an arrestable offence!’.
“Of course [I should have sent him off]. Even in the first minute of a World Cup final, if I’ve seen it clearly enough.”