United begin their campaign against Young Boys in Bern on Wednesday night, with Group H meetings against Juventus and Valencia to come.
Last year, United disappointed in Europe, comfortably qualifying from their group but suffering round-of-16 elimination at the hands of a tough but fallible Sevilla.
Mourinho subsequently complained his squad lacked 'football heritage' - or in other words, Champions League experience - and challenged the club and his players to improve.
Six months on, those comments were put to Mourinho again, but the United manager refused to revisit them, claiming: "What I told last season has nothing to do with this season."
Mourinho did, however, hark back to one of the surprises of last year's Champions League season - Atletico Madrid's elimination in the group stages.
The United manager argued that Atletico's surprise exit from a group containing Chelsea, Roma and Qarabag proves nothing can be taken for granted and that it is "very premature" to think beyond Wednesday's opening game.
"I would focus on tomorrow, first match, and after that on the group phase, one of the most difficult group phases and we have two positions to qualify and our focus must be on that," he said.
"By experience, when a team reaches the quarter-finals, then even the teams that are not favourites, when you arrive in the quarter-finals you smell the distance is not very far and every team starts thinking about it.
"But is too early. We had examples last season of candidates not going through the group phase," he added.
"For example, when Atletico Madrid is knocked out in the group phase that is a very good example that everyone is at risk."
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