The Hatters were hammered 5-1 by Brentford on Saturday.
Clark said: "Very low. It's a tough loss to take. We build this game up so big all week. We were second-best all over the pitch. Full credit to Brentford, we weren't good enough today as a team. I want to say sorry to the fans who were amazing.
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"You've got to go home and look in the mirror as an individual player. We were just second best, that's not like us, we're usually on the front foot, pressing.
"You've got to look at yourself first and foremost, we know we weren't good enough as a team. We've got to be men and bounce back next week. We've got four massive, massive games now."