"I've finished training absolutely disgusted. I can't just sit and take that, the fans need to know," said Clark.
"They pay his wages and what he dished up is the worst effort I've seen in 25 years of professional football."
He continued: "The absolute minimum is to give everything every single day.
"Is it because he hasn't been starting? Is that what you do? Down tools and stop trying? The football club deserves more, the fans deserve more."
"I'm an honest man and, whatever the result [on Saturday], I know I've done the right thing and I'll sleep soundly," Clark added.
"Nikola's future is up to him. There's been no shouting match or bust-up, it's all up to him."