Le Guen demands Cameroon keep focus
Last year's finalists Cameroon started their campaign with a 1-0 defeat to Gabon, face Tunisia in Lubango beforehand.
Gabon are top of Group D on three points, with Zambia and Tunisia joint second with one point.
Cameroon's "Indomitable Lions" will be on the brink of elimination if they do not win but the players looked relaxed at training.
"It's fine by me as long as they stay totally focused," said Le Guen, who helped Cameroon qualify for this year's World Cup in South Africa.
"We intend to approach Sunday's game with all the seriousness it deserves."
Gabon, who were pipped by Cameroon for a World Cup spot in the African qualifying zone, will qualify for the quarter-finals if they beat Tunisia.
The Gabon Panthers have injuries concern with defender Rodrigue Moundounga and midfielders Cedric Moubamba and Arsene Do all doubtful for the game.