Liverpool squandered a two-goal advantage for the second straight game after losing 4-2 to Leeds United earlier this month, raising concerns about their readiness ahead of their Premier League opener against Newcastle United on August 23.
"We played really good (in the) first half, especially the first 30 minutes, but right now probably we don't have much more in terms of the level required to play like this," Iraola told the club's TV channel.
"We won the first two games but the last two, we cannot sustain the level that we want for all the game. As soon as we have made some changes they start to get tired, we cannot sustain that level, so we have work to do."
Captain Virgil van Dijk, goalkeeper Alisson Becker and winger Cody Gakpo were all making their first appearances since the World Cup.
"The problem is right now probably they don't have the energy for 90 minutes and they start to get tired," said Iraola, who joined Liverpool in June after Arne Slot was sacked.
"It will take time with them, we need more training with them. But we can take a lot of positives, but also a lot of negatives from today."
Iraola also said they were taking a cautious approach with new signing Jeremy Jacquet, who suffered a shoulder injury in February and has recently been troubled by a knee issue.
"We were in doubt whether to put him (in the squad) today, we decided not but he should be fine for the next one. It is taking a little bit more time than we expected," he said.