Riga faces a baptism of fire in his first away match as he takes his side to the notorious Den to face their fierce south London rivals.
The match has added spice with both teams battling against relegation, and Charlton's point against Huddersfield on Wednesday night in Riga's first game in charge means a win will see them leapfrog the Lions.
Riga has already been made aware of the significance of the fixture and said: "Even before the Huddersfield game some players and the staff were already speaking about this game.
"So I know that it's very important not only because we have to win and take points but because it is a derby. Day after day I will know more about it."
Coach Alex Dyer also revealed he will be providing the Belgian and his assistant Karel Fraeye with a rough guide to Millwall.
"We will discuss the game and come up with answers," said Dyer.
"I will brief him on the reaction between the two sets of supporters, the crowd, the rivalry.
"They already know a bit about it but by the time I have spoken to them they will know a lot more. We all know what it means to this football club."