"We have decided that this is the best course of action to ensure that both players have a chance of returning to full fitness as soon as possible," West Ham's Head of Medical Richard Collinge told the club's website.
Injuries restricted the 29-year-old Carroll to just 16 league appearances last season while New Zealander Reid played 17 matches.
The duo's unavailability comes as a major setback for new manager Manuel Pellegrini, who is aiming to improve upon a disappointing 13th-placed finish last season.
"This is a big blow for them and us, but we hope to see them back in action as soon as possible," Pellegrini said.
"It does mean that we have lost two players in key areas, though, and we'll continue to work hard with the owners to make further additions to strengthen the playing squad, beyond the five new signings already confirmed."
The London club have signed forward Andriy Yarmolenko, midfielder Jack Wilshere, goalkeeper Lukasz Fabianski, defenders Issa Diop and Ryan Fredericks but will look to further bolster the team ahead of a tough Premier League opener at Liverpool on Aug. 12.
(Reporting By Arnab Paul in Bengaluru; Editing by Christian Radnedge)
