The Ivory Coast international replaced Joe Cole at halftime and struck in the win.
"It is a good feeling," Kalou told chelseafc.com. "I haven't been playing lately and to get in the team and score a goal is very important for my confidence. You can build a lot of confidence for the next game from it.
"That is how you win prizes. You need to have a strong bench, people who can come on and turn a game around. Today I had the chance to come on the pitch and do the header.
"It was a tough game. It is never easy to play in the Cup against a Championship team and in the first half they showed us some tough times.
"We always go for every competition and it is a good feeling to still be in the FA Cup because we know the other top three are out and there is a good chance for us to continue to try to win it again."