Kiernan Dewsbury-Hall struck in the second-half as Chelsea reached the Europa Conference League quarterfinals 3-1 on aggregate.
Maresca admits he had to change tactics during the game to find a way past FCK's defence: "The first and second legs were quite similar.
"The first leg in Denmark we struggled, second half we were much better. The reason why is because we planned both games that way.
"The first game there, we tried to analyse the first half and then push a little bit in the second half in case we needed it. Because we won there, we prepared the game exactly the same way tonight. First half we analysed and saw what we needed to improve."
Maresca continued: "We changed the structure.
"We changed Josh Acheampong not because he was not doing well – he was doing very good the first time he was doing a different job – but because we needed to change the structure. You need patience.
"We were attacking with five players and we needed to attack with six. The second half was much better and we deserved it over two legs."