Mourinho, whose team were held to a first leg 1-1 draw at Rostov on Thursday, said he would rotate his squad for the Chelsea clash but not fill it with youngsters from interim under-23 manager Nicky Butt's team.
"It is not Chelsea's fault that we were given this Monday match, so we have to make changes for sure... but we cannot go to Stamford Bridge with a Nicky Butt team," the Portuguese said after the match in Russia.
"We cannot go with Nicky Butt's team. Manchester United is too big. Manchester United is the winner of the competition."
United, who have not won at Stamford Bridge since 2012, were beaten 4-0 on their last visit to Chelsea in October, Mourinho's first visit to his former club since he moved to Old Trafford at the start of the season.
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(Reporting by Aditi Prakash in Bengaluru)
