Heskey's urged to steam ahead.
O'Neill fears the England striker's injury and illness niggles are stopping him from producing his best.
"I take the point that he had completed a couple of games recently and that was good news psychologically for him," said O'Neill.
"I think you need to build up a head of steam. It's a wee bit like Carew as well.
"People say 'well, when do we get these opportunities'? Every minute that you're on the pitch is vital."
Heskey, Gabby Agbonlahor and Stephen Warnock are all in contention to play tomorrow after overcoming injury. O'Neill likened Heskey's situation to that of youngster Fabian Delph, who is trying to achieve match fitness despite being in and out of the team.
"While I accept that it can affect someone so young, it can certainly affect other people who are a lot more mature. I agree you need to get that head of steam up," he added.
Meanwhile, O'Neill believes Villa's Cup form has diverted attention away from their Champions League push.
He added: "In many aspects the spotlight's possibly on us maybe a little bit more even because of our exploits in the Carling Cup.
"I just think that because we're still involved in the cups perhaps other people maybe give our league positioning less attention."
Fears: Emile Heskey.