"I'm happy with the six goals, but totally unhappy with the way we gave three goals away. That has to improve and the players know that," the 67-year-old Dutchman was quoted as saying by the Sunderland Echo.
"I am wondering if it is to do with concentration, sharpness -- it cost us three goals, and that's a worry. And it's not the first time," the former Netherlands manager added.
The Black Cats have been heavily linked with Man City's Belgian international defender Jason Denayer, but Advocaat played down the move.
"There has been talk about the young guy from Manchester City, but he is going to another club," said Advocaat.
"We are still looking, but when (Younes) Kaboul is fit, he's a very good player."
(Reporting by Shravanth Vijayakumar in Bengaluru; editing by Amlan Chakraborty)
