The match took place at the home of rugby league side Warrington Wolves, and was beamed out on live on satellite TV.
Toon's 'Worst Performance'!
3,171 watched the game, as this fixture always attracts the crowds, but Clark said his young side appeared to suffer stage fright.
Clark: "It was probably our worst performance of the season.
"We didn't compete the way we should have and we didn't pass the ball the way we can.
"Perhaps people may think I am setting standards too high, but they are the standards that you will need if you want to play in the first team.
"Jonny Godsmark got us a goal just before the break and there was a little improvement, but I am disappointed.
"These are young players, though, and I hope they can learn from their experience.
"For Celestine (Babayaro) and Shola (Ameobi) it was important they got some game time, and that was the most positive thing of the night for those two."
Ameobi (pictured) has suffered with a groin problem, but he played the full 90 minutes last night.
Kewell head home in the first half to open the scoring, but Godsmark scored from eight yards three minutes before the break.
Liverpool won the fixture when Jordy Brower made the most of goalkeeper Fraser Forster's mistake to score the winner.
LIVERPOOL: Martin, Darby, Insua, Kelly, Huth, Spearing, Plessis, Lindfield (Pacheco 82), El Zhar, Kewell (Brouwer 46 [Putterill 89]), Leto.
UNITED: Forster, Bertram (Francis 61), Babayaro (Lough 67), Doninger (Danquah 81), Edgar, Morris, Lua Lua, Pattison, Ameobi, Godsmark, Troisi.