The Portuguese endured one of the heaviest defeats of his managerial career - and certainly the most surprising - as his Roma side were thrashed 6-1 by Bodo/Glimt in the Europa Conference League.
Roma trailed just 2-1 at the break after the Norwegian champions struck through Erik Botheim and Patrick Berg's curler, while Carles Perez replied with a clinical effort.
Mourinho made three changes at the start of the second half and another two on the hour mark after Bodo/Glimt added a third with Botheim tapping in from close range.
But there was more misery for Mourinho and Roma as Ola Solbakken's fine solo goal made it 4-1 before Amahl Pellegrino added a fifth and Solbakken claimed his second.
It is the first time a Mourinho side has conceded six or more goals in a single game in what was the 1,008th match of his coaching career.
It sits alongside the 5-0 loss his Real Madrid side suffered against Barcelona in 2010 as one of the biggest defeats for Mourinho as a manager.
"I decided to play with this line-up, so the responsibility is mine," he 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."