It was the first time in 1,008 games as a coach that a Mourinho side conceded six goals or more.
"I decided to play with this line-up, so the responsibility is mine," the 58-year-old told Sky Sport Italia. "I did it with good intentions, to give an opportunity to those who work hard and to rotate the squad on a synthetic pitch in cold weather.
"I never disguised the fact we were a squad with real limitations. We have 13 players who represent one team, the others are on a different level. We lost against a team that showed more quality on the night. It's that simple."
Erik Botheim gave the hosts the lead after only eight minutes before Patrick Berg doubled the lead on 20 minutes. A much-changed Roma side, which featured United States international Bryan Reynolds in defence, was given hope when Carles Perez made it 2-1, and Mourinho introduced Henrikh Mkhitaryan, Bryan Cristante and Eldor Shomurodov at half-time in an attempt to rescue the game.
Ola Solbakken scored twice and Amahl Pellegrino was also on the scoresheet to complete a miserable night for Mourinho.
The defeat may not have been quite as painful for Mourinho as Real Madrid's humiliating 5-0 loss to Barcelona in 2010 but Roma, who started the Europa Conference League with two wins, are now second in the group on six points, one behind Bodo/Glimt.