The Hoops are away at Atletico Madrid in their first group stage match in Europe's second tier club competition but Mulgrew says that the Scottish Premier League meeting with their Glasgow rivals takes precedence.
"Europe was always a bonus this year," he said. "The priority was always going to be the league.
"This is a big game for us and a great experience for all the players but I think Sunday is a bit more important.
"It is a massive week, there are going to be a lot of massive games this season and we all look forward to them all."