With the game deep in stoppage time, City's Jack Grealish was through on goal when referee Simon Hooper blew his whistle for a foul on Haaland in the build-up, having previously waved play-on.
Several City players surrounded the referee over his decision to halt play and the club were later charged by the FA over their conduct.
"Yeah, it is an officiating mistake," Webb said on the 'Match Officials: Mic'd Up' TV programme.
"On this particular occasion, (the referee) sees a pretty strong, reckless foul challenge by Emerson Royal on Haaland. It’s in midfield, and the ball comes backwards.
"He does the hard work, he just hesitates a moment to see if an advantage is possible. But he recognises in his mind, thinking it through, that it was a strong challenge, it was reckless, he is going to have to caution."
Webb said Hooper was "devastated" over the mistake.
"At ground level, it’s not quite so easy to see the way that things are playing out as it is from an elevated view," he added.
