Chelsea's only reply to Finn Miika Koppinen's goal in the 24th minute was Andriy Shevchenko's header off Florent Malouda's cross in the 53rd minute.
"The story of the match is ... we couldn't score two goals in 20 chances and we could not defend a set piece [on Koppinen's goal]," said Mourinho.
Chelsea was without the injured Frank Lampard and Didier Drogba for the Rosenborg game and should be without them again on Sunday when it travels to Old Trafford to face Manchester United in their EPL showdown.
The Blues' offense has been woeful this season. They are scoreless in their last two EPL games and have only scored three goals in five games over the last month.
"For me," Mourinho added, "the drama is we play as an attacking team, dominating games, and we score one single goal in three games."