The Ivory Coast international scored the only goal of the first leg in Lisbon as the Blues took the advantage in the Champions League quarter-final.
And Di Matteo told Sky Sports 2: "He's come into the team after a long spell out, he got the goal and had a great game.
"I didn't think it was a gamble but sometimes in life you have to take a little risk.
"With the fact we are playing all these games, it was important to select a team that had the energy for a high-tempo game. It's terrific, we couldn't have hoped for a better result.
"We went close on a number of occasions and kept a clean sheet, we looked good defensively again. They had some set-pieces and the keeper had a few saves to make but they were all from long range."
Kalou, who converted Fernando Torres' cross in the 75th minute, added: "It's a good result, the away goal is very important.
"You want to play every game but you have to respect the manager's decision. He gave me a chance tonight [Tuesday] and I have to show it on the pitch, so I'm happy to get the goal.
"The most important thing is we are ahead and now we have to be ready for the next game at home."