An own goal from Wessam Abou Ali on 49 minutes got the Brazilian side off the mark before Jose Manuel Lopez sealed the 2-0 win with his goal with 59 minutes on the clock.
Riveiro only took charge of the Egyptian side in May as a replacement for Marcel Koller.
He secured a goalless draw with Inter Miami in Al Ahly's first Club World Cup game, who now need a result against Porto on Tuesday if they are to have a chance of progressing to the knockout rounds.
The former Orlando Pirates boss said on iDiski Times: "One free kick and it was 1-0, then we lost the ball, and he scored another goal. I think the game was even before that, but they scored the goals."
The match was halted in the second half after reports of "severe weather" before ultimately resuming after a 40-minute delay.
"The match was beyond our capacity. We tried to come back after the weather interrupted with a new style of play, but we failed to create enough opportunities," explained the Al Ahly coach.
Borussia Dortmund head coach Niko Kovac has acknowledged the challenge that awaits his side when they face South African champions Mamelodi Sundowns in their upcoming FIFA Club World Cup Group F fixture at TQL Stadium in Cincinnati on Saturday.
Al Ahly striker Trezeguet promises to fight to win their final Group A clash against Porto despite a 2-0 loss to Palmeiras at MetLife Stadium on Thursday.
Divine Lunga admits it is a strange feeling heading into their Club World Cup clash against German giants Borussia Dortmund as underdogs.
Manchester City coach Pep Guardiola heaped praise on Wydad Athletic following his side's 2-0 Club World Cup win over the Botola Pro 1 side on Wednesday.
