A shorthanded Spurs side failed to muster much of a threat in the Champions League semi-final first leg, as Donny van de Beek's first-half strike put Ajax on the verge of their first final since 1996.
Despite injuries and suspension robbing Tottenham of several key players including Harry Kane and Son Heung-min, Eriksen was still fuming with the way his side performed – especially in the first half.
“We were under-par, we didn’t play our best at all," Eriksen, who played for Ajax between 2010 and 2013, told BT Sport .
"In the first 20 minutes, we were ball-watchers. We need to change it up a lot for the next game.
"We made them look a lot better than they are. Of course they're a good side but I think we helped them on their way to give them the feeling that they can control things, which was our fault.
“At the start, the system didn’t matter. The change gave us a different way to press, to play more direct. But we were still far from where we know we can be.
“No one wanted to play the first half we did. Everyone knew we didn’t compete. We still lost, but we were better in the second half.
"Second half was different, we played more direct and we hurt them a lot more, but we needed the last touch."
Though Tottenham were shorthanded prior to the match and then lost Jan Vertonghen to a head injury in the first half, the Denmark international wasn't interested in injury excuses.
“We can’t keep talking about injured players. In a semi-final it doesn’t matter who plays, we have to step up," Eriksen said.
Eriksen felt his side were fortunate to see David Neres hit the post in the second half, which will give Spurs some encouragement heading into next week's second leg at the Johan Cruyff Arena.
"We’re lucky they hit the post and hopefully we can turn things around in Amsterdam," the 27-year-old said.