Kaka was given a second-half introduction by coach Jose Mourinho.
And Karanka remarked: "Everyone is seeing the improvement in Kaká. We were the first to say that if he stayed, as last year, he would have matches to show what he has.
"When a player works as he does when they get these matches and these opportunities it is reflected on the field, which is what we all want. We want all players to be at 100%, which in the end is more difficult for us but it is the best for the team."