Sunderland manager Sam Allardyce said he must act in the January transfer window after Saturday's 4-1 defeat to Manchester City.
Raheem Sterling opened the scoring after just 12 minutes before Yaya Toure and Wilfried Bony added two more in quick succession.
The outstanding Kevin De Bruyne claimed a deserved fourth early in the second half before Fabio Borini netted a Cats consolation.
Allardyce, who made six changes for the game, said: "We have got players who are saying they deserve a chance in the team and, 'Why am I not playing when the team is struggling?'
"Everyone has had a chance now and obviously they have not done as well as they should have done. Now it is my time to make some big decisions about that squad, and think about the January window and ask 'Who can we bring in?' and 'Who am I going to move out?'"
Allardyce, who was without centre-back Younes Kaboul, felt his defence was opened up too easily by City.
He said: "Their three goals in nine minutes were three goals we could have done a lot better with, particularly the first and the third.
"In the last three games we have gifted teams goals and found it difficult to come back. As much as you change the side around, which is what I have been doing since I've been here, and think you are going in the right direction, clearly we are slipping back into the danger zone and it is hard work to think now what my best team is.
"Until we start getting better defensively we are going to struggle to get results. That is our big problem. As a team we do not defend as well as we should do."