The young Blues defender was stretchered off after landing awkwardly in the second-half at Elland Road, with Clark confirming the American has broken his left leg.
The Blues boss told bcfc.com: “I think the early diagnosis is a break to both the fibula and tibia, so it took the gloss off a fantastic performance by us today. His young team-mates and friends were distraught on the pitch; in tears. I tried to get them refocused and I just said to them ‘let’s try and win the game for Will’ but we couldn’t do it.
“We were brilliant and I thought we deserved to win the game, but we’ve lost a good young player who’s a magnificent kid. We love him to bits at the football club. He’s so honest and genuine and he’s a good pro.
“It’s not actually his proper position right-back. He’s played his entire career as a young lad at centre-half and he must be in double figures for appearances at right-back for us now. He just gets on with it. We’re going to give him the best care possible and get him back fitter and stronger next season – and he’ll have the support of everybody at the club.”
Blues secured a FA Cup with Budweiser third round replay against Leeds United, and Clark was pleased with the way his players performed.
He added: “I thought we played some excellent football and created some really good chances. On another day we should be running out comfortable winners and be in the hat on our own, instead of having to go to a replay that we’ll hopefully win.
“That’s not even the job half done because Neil’s (Warnock) got a good, powerful team so it’s going to be just as tough at our place.”