McDermott's men made quite hard work of reaching the second round, falling behind to League Two Chesterfield at Elland Road.
But Michael Brown levelled with a fine goal after 28 minutes and Poleon then produced a good finish of his own three minutes later, a goal which proved to be the winner.
"Dominic has lots of potential," said McDermott. "We are trying to turn him into the all-round player he can be.
"We haven't played well tonight but we won the match and that is very important.
"I must confess I didn't enjoy it, we made seven changes and that can backfire on you.
"Some big teams have gone out of the competition and we are not one of them, so that is pleasing. Once again we fell behind and that is something we have to address but the boys kept going and got their reward."
McDermott confirmed after the game that Leeds had rejected two bids for Scotland forward Ross McCormack.
"Ross is staying here," said McDermott. "He wants to stay and we want to keep him."
McDermott also denied making an offer for Matt Taylor of West Ham.
Chesterfield take the positives
Chesterfield assistant manager Leam Richardson was rightly pleased with his team.
"We came here to give a good account of ourselves and we did that," he said. "The result was disappointing but there were a lot of positives to come out of the game.
"We are trying to gel as a team and make small steps together. We controlled a good proportion of the game, which breeds confidence.
"Our young players will come on by leaps and bounds, now we have to raise our game against Cheltenham on Saturday."
Chesterfield's star performer Gary Roberts felt he should have had a penalty just after the break when he was brought down by Zac Thompson but referee Carl Boyeson decided the offence took place outside the box.
Richardson said: "I thought it was a penalty but I would have to see it again to be sure. The officials have a tough job in those circumstances."