The Carabao Cup holders trailed inside seven minutes as Marmoush exchanged a quick one-two with Phil Foden before Dan Burn's sliding tackle saw the ball bounce kindly off the Egyptian forward before landing in the net.
Magpies head coach Eddie Howe made three changes to the starting XI as Aaron Ramsdale, Sven Botman and Nick Woltemade replaced Nick Pope, Harvey Barnes and Anthony Elanga respectively.
Newcastle aim to overturn a two-goal deficit at the Etihad Stadium, following a 2-0 defeat at St. James' Park 22 days ago, and reach a third Carabao Cup final in their last four seasons, setting up a final showdown with Arsenal at Wembley Stadium.
Backed by over 5,300 away fans tonight, the Magpies' last away win against Manchester City was recorded in October 2014 as goals from Rolando Aarons and Moussa Sissoko sealed a memorable 2-0 victory in the League Cup fourth round.
**Newcastle United:** Aaron Ramsdale, Kieran Trippier (c), Lewis Hall, Sven Botman, Sandro Tonali, Anthony Gordon, Malick Thiaw, Nick Woltemade, Joe Willock, Dan Burn, Jacob Ramsey.
**Substitutes:** Nick Pope, Yoane Wissa, Harvey Barnes, Will Osula, Anthony Elanga, Jacob Murphy, Alex Murphy, Leo Shahar, Sean Neave.