Sunderland boss David Moyes said he "couldn't believe" the standard of his side's defending in their 3-0 Premier League defeat to Everton on Monday.
Having matched the visitors during the first half at the Stadium of Light, Sunderland collapsed to a third defeat in four league games thanks to a hat-trick from Romelu Lukaku in the space of 11 second-half minutes.
The Belgium international twice headed in unmarked from point-blank range before slotting in a third on the break after Kevin Mirallas split the home defence with a simple first-time pass.
And Moyes is eager to assess what went wrong as soon as possible in order to avoid a similar result when they travel to Tottenham on Sunday.
"We were really poor in the second half, but I thought, in the first half, we showed some good things. We had one or two opportunities ourselves," he told Sky Sports.
"But we weren't in the right positions to defend crosses in the second half. For two of the goals from crosses, I couldn't believe we didn't have defenders in the middle of the goal, especially against Lukaku.
"We're putting together a new team, it will take time. But I was really disappointed with the last 30 minutes especially.
"There have been a couple of games where we've been right in it right at the end, but the minute we lost a goal tonight, we never looked like getting back into it.
"I thought we gave them away really cheaply, we didn't do basic defending and we got punished. You have to give Everton credit, they picked people out with a couple of really good balls. We just weren't in the right defensive positions, which is something we work on regularly. I can't wait to get to them and ask why they weren't doing it.
"We've got to try not to concede any goals, we had some decent play going forward at times and we had a young side out. But that's no excuse - when you're in the big league, you've got to step up to it."