In the end it didn't matter as City were easy winners, making it five in injury time as Sterling slotted in his hat-trick to make it five.
But Sterling grabbed his second in the 75th, timing his run for Mahrez's ball over the top before lifting it over Fabianski, and Aguero was given a second chance to take his penalty after Fabianski saved his first effort.
It was a game in which VAR was significant. VAR is here to stay.
The Premier League champions romped to a 5-0 win over the Hammers at the London Stadium and got their league title defence off to a wonderful start.
Manuel Pellegrini's men will attempt to reverse a seven-match losing streak against City in all competitions. When Guardiola's side found extra gears the visitors were able to put the result beyond any doubt.
West Ham were momentarily heartened and Diop could have pulled one back but he headed a Robert Snodgrass free-kick over, before Javier Hernandez was denied at point-blank range by Ederson.
Jesus was nearly gifted a second before half-time when Issa Diop sliced his clearance but the striker could not lift the ball over Fabianski.
The visitors built on their lead after the interval when Mahrez collected possession in midfield and shuttled possession on to the highly influential De Bruyne.
Brighton, who finished two points above the relegation zone last season, got off to a flying start under new boss Graham Potter.
On 25 minutes, City made the breakthrough and it was a familiar goal that West Ham have been conceding all pre-seaso. However, the VAR adjudged Sterling had been offside.
Local boy Billy Sharp came off the bench for Sheffield United to earn a share of the spoils in their first Premier League game since 2007 as they drew 1-1 t Bournemouth. Aguero went up again for the retake and made no mistake at the second time of asking to make it 4-0.
City doubled their lead through Sterling and, after Jesus had seen his potential second chalked off by VAR, technology ensured the England international's second did count before he added a third late on, following Aguero's penalty.
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