He told a news conference on Friday that one of the reasons the north-east club struggled last season was a lack of competition.
"We have brought in players who improve the squad," the experienced Dutchman said. "Too many players last season were very sure of their position and that is more difficult this season.
"I don’t want to discuss positions, but we still need one or two players for the team, not for the bench.
"The most important thing is that you have competition, that everybody has to fight for their places."
Sunderland had hoped to sign the Queens Park Rangers midfielder Leroy Fer but he was reported to have failed a medical.
Midfielder Yann M'Vila, who did join this week, will not play against Leicester City on Saturday but Advocaat said he is hoping for great things from Jeremain Lens, signed from Dynamo Kiev.
"We had a problem last season making goals and also creating chances, so something was wrong in both midfield and up front and that's why we bought Lens." he added.
"He is a very good addition who has pace and can score goals. If we can play attacking football then we have the players who can do that."
(Reporting by Steve Tongue, editing by Pritha Sarkar)
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