Grealish had begun the first-leg playoff brightly, but was forced off in the first-half, with Shearer insisting a lengthy VAR delay the source of the issue.
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A VAR review involving City striker Erling Haaland lasted four minutes and Shearer, in his pundit's role, said: "I was just saying about how well of a start he had made to the game.
"It would be such a shame if he had to go off. He's played a huge part in the goal.
"It certainly doesn't help when you've got elite athletes that are standing around for almost four minutes, it cannot help you or your body.
"It's not acceptable that players are having to wait around for that long."
Grealish was eventually forced off and replaced by Phil Foden on 29 minutes.