Reserve horse and rider combination Robert Smith and Talan will replace Fletcher in the team.
"We are very disappointed for Tina and Lady Harris who owns Hallo Sailor," Hoekstra said.
"However, we are extremely fortunate that we have a world class rider in Robert Smith to bring into the team and Talan has shown excellent form on a recent trip to Calagry."
Smith joins Scott Brash riding Stanley Brash's Intertoy Z; David McPherson riding his own Chamberlain Z and Michael Whitaker riding Beatrice Mertens GIG Amai II to complete the showjumping team.
The Games take place in Kentucky, USA from September 25 to October 10, with the showjumping competition taking place between October 4 and October 9.