Sunderland have already signed right-back Adam Matthews from Celtic and centre-back Sebastian Coates on a permanent deal from Liverpool, but Advocaat insists there is still plenty of work to be done in the transfer market.
“We discussed with Lee and the president [chairman Ellis Short] at the end of last season, we need more competition in the squad,” Advocaat told the Sunderland Echo.
“We have two new players with Coates and Matthews, but hopefully we will still get some other ones in as well.
“We still need three, maybe four [signings].
“It’s still very quiet in the transfer market in England, which is strange.
“We still hope we can get three extra players in the squad, which will make everything a little bit easier. Some players will go out, too.
“But it is too early to mention about names – you will have to wait and see.”
Strengthening the attack is a priority for a team that only scored 31 goals in the Premier League last season, but Advocaat acknowledges the difficulty in signing top players on a budget.
“We did not create a lot of chances all season, and we did not score a lot of goals,” the Dutchman added.
“But the providing to the strikers was also not good.
“It also depends on what players we can get, and we are working on that.
“And it depends on the players who will come, because we are not Manchester United or Manchester City, and we don’t have unlimited money.
“So we have to be very careful about the players we bring in.”