Amorim has come in from Sporting CP with the task of bringing United back to the very top.
Shearer knows this will be a huge challenge, given the managers who have failed since Sir Alex Ferguson retired.
Speaking to Betfair, he said: "Ruben Amorim officially started his new job as Man Utd manager and in an ideal world, he’ll start climbing the Premier League table immediately and clinch a European spot.
"You never get a free pass when you become the new manager of Manchester United, but you might get away with one or two things because there's so much to be done at the football club and so much to be changed.
"It may take a little bit of time for Ruben to implement his own ideas and own system on some of the players because they'll probably try a system that they haven't played before.
"Whilst you don't want to make excuses for anyone, the damage that's been done over the years at Man United and how hard it's been for them, you'd understand if it takes a bit of time for Amorim to get it right."
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