United ripped the League One outfit in half at Old Trafford thanks to doubles from Marcus Rashford, Alejandro Garnacho, and Christian Eriksen and a penalty from Antony.
Clarke was not happy with his side who were expected to lose but not in such a fashion.
“If I ever hear of my players telling me Premier League players are poor I'll be giving them a slap because we were miles off against a team that can absolutely destroy you when you're not at it
“They're a quality team but we're very disappointed in our performance. We haven't done ourselves justice. That was my fear tonight coming here.
“A lot of our players had an off-day. Maybe one player, Mael (de Gevigney), probably comes out of the game with credit, but not for us.
“I know it's a big occasion against a very, very good team, I get that, and they turn it on against Premier League teams, but it's a huge wake-up call for me, to the players, to understand what it takes to be a player.”