Advocaat has completed five new deals since making his own role at the Stadium of Light permanent at the end of last season, and is eyeing at least two before the end of the transfer window.
The Dutchman believes many of his side's problems during the last campaign came about because a number of players were able to rest on their laurels with their places in the starting line-up assured.
Advocaat said: "We have brought in players who improve the squad
Too many players last season were very sure of their position and that is more difficult this season.
"We have brought in five players and Connor Wickham has gone, and we still hope to bring in one or two players more.
"The most important thing is that you have competition - that everybody has to fight for their places
You can see now in training that that is improving."
Jeremain Lens is expected to join Jermain Defoe up front in his new side's season-opener at Leicester on Saturday, but there will be no place for fellow new boy Yann M'Vila.
The French international is short of match fitness after arriving on a season-long loan from Rubin Kazan this week, and is instead expected to start with the reserve squad.
Advocaat added: "He is a player who has been very much in form for France
He didn't do much for the last two years - he went to Russia for a lot of money - but he knows that Sunderland have given him the chance to do well."
The arrival of Lens will raise hopes among home fans that the club's widely criticised negative approach under former boss Gus Poyet will give way to more permanent examples of the attacking football shown during another last-ditch survival bid.
"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." added Advocaat.
"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."
Steven Fletcher could challenge for a starting place despite missing the last pre-season friendly against Hannover 96 with a slight knock, but defender Wes Brown is likely to be ruled out.
Source : PA
Source: PA