The 32-year-old was sent off midway through the first half at Upton Park after bundling into West Ham midfielder Mo Diame to concede a penalty.
McGregor had received treatment on the field immediately after the challenge and Bruce confirmed after the game that there are worries over the damaged caused.
"McGregor is in a bad way, kidney damage," the Hull boss said.
"A specialist is on his way to see him, it is a contusion on his kidney. We're hoping it's not ruptured. He's gone to hospital and he'll be staying overnight."
Substitute goalkeeper Steve Harper could not prevent Mark Noble putting the Hammers ahead from the resulting penalty but Hull rallied with 10 men and equalised through Nikica Jelavic early in the second half.
West Ham secured the win in fortuitous circumstances as Hull defender James Chester turned Guy Demel's cross past Harper just six minutes after the leveller, with the hosts able to hold on for the three points.
- Sports & Recreation