The Argentina international, boasting 18 goals in all competitions this term, drops out of the starting lineup as one of four changes from last time out.
In the absence of a recognised striker, Raheem Sterling is expected to start through the middle for the Citizens, as they look to maintain their unbeaten domestic run stretching back to April.
Converted striker Reid heads into tonight's semi-final with 13 goals to his name this campaign, playing a key part in the Robins' surprise run to the last four of the competition.
Johnson makes eight changes in all, reverting back to his strongest starting lineup, and there is also a place on the bench for new signing Liam Walsh.
Manchester City (4-3-3): Bravo; Danilo, Stones, Mangala, Zinchenko; Gundogan, De Bruyne, Toure; Bernardo, Sterling, Sane
Subs: Ederson, Walker, Aguero, Delph, Fernandinho, Diaz, Adarabioyo
Bristol City (4-4-1-1): Fielding; Wright, Flint, Baker, Magnusson; Brownhill, Pack, Smith, Bryan; Reid; Paterson
Subs: Steele, Vyner, Kelly, Eliasson, Walsh, Engvall, Taylor
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