Goalkeeper McGregor faces six months on the sidelines due to a back injury. The Scottish international will reluctantly undergo an operation on an injury that he has been carrying for some while.
"Surgery was always going to be the last resort with Allan because it's a horrible, complex operation," Hull manager Steve Bruce said.
As for Alex Bruce, he has injured his Achilles and will miss the first four months of the season.
"They're severe injuries and you don't want that at this particular time," said Bruce Sr.