Manchester United boss Louis van Gaal says those who criticise their demotion to the Europa League from the Champions League are "living in the past".
United, who play Liverpool in the last 16 on Thursday, are in Europe's second-tier competition for only the second time in the past 20 seasons.
"It was another time. You live in the past," said Van Gaal of United's regular recent appearances in the Champions League knockout stages.
"It happens in football. It's normal."
Manchester United have won three European Cups - most recently in 2008 - while Liverpool have lifted the trophy on five occasions.
After defeat by Wolfsburg in December condemned United to the Europa League rather than the knockout stages of the Champions League, former captain Rio Ferdinand said the club should be "embarrassed," while ex-striker Michael Owen claimed they had gone backwards under Van Gaal.
But the United boss insists it is inevitable that the two clubs' fortunes change after a period at the top of the European game.
"It's not normal that one team dominates for 20 years in a row as champions," he added.
"You are talking about Manchester United and Liverpool, and we are happy to play in the Europa League, but City, Chelsea and Arsenal are still in the Champions League," he said.
"Six clubs are representing the Premier League. You have to be very proud of that because not many countries have so many clubs participating at that high a level."
Manchester United are three points and two places off the Premier League's top four, but could qualify for next season's Champions League by winning the Europa League in this campaign.
Van Gaal's predecessor as United manager David Moyes was sacked by the club two days after defeat by Everton in April 2014 confirmed the club would miss out on Champions League football for the first time since 1995-96.