Leigh Griffiths came off the bench to head home the winner in the closing minutes against the Norwegian champions.
Celtic's next match in the section is away to Salzburg - who were 3-2 winners against RB Leipzig in Germany - on Thursday October 4.
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Celtic defeated Rosenborg 1-0 in their Europa League Group B opener in Glasgow on Thursday night.
Leigh Griffiths came off the bench to head home the winner in the closing minutes against the Norwegian champions.
Celtic's next match in the section is away to Salzburg - who were 3-2 winners against RB Leipzig in Germany - on Thursday October 4.