The Norwegians stunned the Blues by taking a 24th-minute lead through Miika Koppinen at Stamford Bridge, with Andriy Shevchenko snatching a 1-1 draw for the toothless hosts.
Mourinho spoke of his alarm at Chelsea's failure to score goals after the match, as well as his concern about heading to Manchester United that weekend without injured stars Frank Lampard and Didier Drogba.
But it would be Avram Grant leading the side out at Old Trafford, with the Portuguese leaving the club by mutual consent just two days after the Rosenborg clash.
Chelsea said in a statement: "Jose did not resign and he was not sacked.
"What is clear, though, is we had all reached a point where the relationship between the club and Jose had broken down.
"This was despite genuine attempts over several months by all parties to resolve certain differences.
"The reason the decision has been taken is that we believed the breakdown started to impact on the performance of the team and recent results supported this view.
"We did not want this to continue or affect the club further."
Mourinho won two Premier League titles during that first stint along with an FA Cup and two League Cups.
The 57-year-old returned to the club in 2013, winning a further Premier League and League Cup before being sacked in December 2015.