The midfielder pulled up in last week's 2-1 win at Crystal Palace and could be sidelined for up to four weeks.
Reyna - the USA captain - could now also be forced to sit out his country's two World Cup qualifiers against El Salvador and Panama in October.
His injury leaves City short of midfield options with Trevor Sinclair and Steve McManaman also sidelined, though Kevin Keegan confirmed McManaman could return to face Southampton.
Paul Bosvelt therefore looks set to partner Joey Barton - who signed a new three-year contract in midweek - in centre midfield.
With defender David Sommeil also out, Richard Dunne will return after sitting out Tuesday's thrashing of Barnsley, while Nicolas Anelka and David James are also back after being rested.