I have found that sometimes just one signing can disrupt a team even if they are not a disruptive player - e.g. when at Arsenal I signed another DM as backup for Gilberto, thus upsetting denilson and Diaby who presumably fancied themselves for a run in the team. "too much competition" apparently.
I've no idea if players being unhappy about that sort of thing impacts how well your team plays though | 
