Ireland was subjected to a record defeat against England at Twickenham, Manu Tuilagi and Maro Itoje running the show while Joe Cokanasiga scored twice in a 57-15 home win.
The visitors had the opportunity to move to the top of the world rankings with victory but will instead go into next week's match with No.1 side Wales reeling from an awful defeat.
Ireland was extremely sloppy, with its lineouts woeful, and captain Best acknowledged the need for considerable improvement in the coming weeks.
"We can be a lot better and we have to be a lot better," he said. "That is not up to the standard that we set ourselves.
"Quite frankly, it is hard to describe it without using a lot of profanity. We're going to have a look back at what went wrong. It will not be pleasant, but we're going to have to get better.
"We're nowhere near where we need to be and the only positive we can take from it is that it's the middle of August and not the middle of September. We need to improve significantly across the board."
Owen Farrell was understandably more upbeat about England's display, but promised there is still more to come ahead of the tournament in Japan.
He said: "We are still building. I think there is a lot left in us.
"We are still looking forward to playing our best rugby. That, for me, is very exciting.
"We are building. The most exciting thing is that we have got a lot of good rugby in front of us, hopefully, and we've got to make sure it keeps going that way."