Reading midfielder Steve Sidwell and Bayern Munich's Peruvian striker Claudio Pizarro have already arrived for nothing, and they will joined PSV defender Alex - who is owned by Chelsea.
Mourinho said: "For the next season Chelsea have told me there is not much of a budget for reinforcements.
"The money will be practically nil and we will not be paying for transfers. But I don't think that investment is necessary because Chelsea already have an able and successful team as long as we don't have injuries. That is what hurt us last season.
"Even without big investments, Chelsea will be fighting with [Manchester] United. I am convinced that we will be the best next season."