Barcola made his first start for Liverpool in the competition and impressed before being replaced shortly after the hour mark. The France international had twice gone close to scoring before leaving the field.
Iraola confirmed there was no injury concern, with the forward simply suffering from cramp.
"No, it is cramp. It is just cramp, there is no injury," Iraola said.
The Liverpool boss explained that Barcola's minutes are still being carefully managed after the player missed pre-season with Paris Saint-Germain.
"With Bradley, it's about building his physical condition," Iraola said. "We didn't really know until now how much he can play because he hasn't had any minutes in pre-season for PSG.
"He played 30 the other day and today we had some question marks about how many minutes he can give us. Probably now we know."
Iraola was pleased with Barcola's attacking contribution despite the 23-year-old failing to find the net.
"I'm very happy because also he has had two very clear chances against the 'keeper," he said. "He hasn't scored, but I want him there.
"There is where he has to make the difference, arriving to those spots, those positions, and I'm very happy."
Liverpool were also without Cody Gakpo against Atletico after the Netherlands international missed training on Tuesday.
Iraola said Gakpo has no serious injury but admitted it remains uncertain whether he will recover in time to face Fulham on Saturday.
"He doesn't have a proper injury, but he finished with niggles in both of the adductors the other day," Iraola said.
"We expected to have him for today, [but] it hasn't been possible. It's not a big thing, but the problem is we play here in two days and a half."
Iraola will assess Gakpo during the recovery period before deciding whether he can feature against Fulham.
"I hope so, but I don't know," he said. "Let's see how Bradley has finished, how Cody is and if he can give us some minutes."
Kerkez was another player forced off against Atletico, with Kostas Tsimikas replacing him in the 88th minute after receiving treatment.
However, Iraola confirmed the full-back was also suffering from cramp rather than an injury.
"Milos and Bradley were just cramping and were just tired," he said.
The manager was pleased that both players had pushed themselves to their limits before being substituted.
"I always want to do subs because of this, because you've given everything, you don't have any more, perfect," Iraola said.
"We do the subs, not because you haven't run and I have to sub you. I prefer when you've done everything you can and then we sub you because you cannot run anymore."
Liverpool now have little time to recover, with Fulham visiting Anfield in the Premier League on Saturday.
Manchester United surrendered their advantage to draw 1-1 with Tottenham Hotspur at Old Trafford on Saturday evening, missing the opportunity to claim their second Premier League win of the campaign.
Chelsea bounced back from their defeat before the international break with a convincing 5-1 Premier League victory over Bournemouth at Stamford Bridge, as Jordan Henderson scored his first goals for the club.
Arsenal came from behind to secure a hard-fought 2-1 victory over Leeds United at the Emirates Stadium on Saturday, showing resilience to claim maximum points after falling behind in the second half.
Cole Palmer has signed a one-year contract extension with Chelsea, keeping the England international at Stamford Bridge until 2034.
