The 19-year-old collided with Luciano Becchio after the Leeds forward had equalised Wade Elliott's opener for Birmingham.
The injury will rule him out for the rest of the season and Clark, who heard the player's leg break from the bench, said the team were deeply upset by the injury.
"It took the gloss off a fantastic performance," said Clark. "Will's young team-mates and friends are distraught, they were in tears out there.
"I said, 'Let's try and win the game for Will'. We couldn't quite do it but I thought we deserved to do so.
"We will give him as much care and support as possible and have him back fit and strong for next season.
"We should be celebrating a fantastic performance by my young players, but this is a real downer.
"I heard the snap and was hoping it was a shin pad. It was an innocuous one with two players going for the ball."