Erik ten Hag has issued new signing Joshua Zirkzee with an urgent warning and laid out a timeline for his progress.
United signed the Netherlands international from Bologna for £36.5million on July on a five-year deal.
The striker, 23, has only started training with his team-mates in the past few days.
However, a hamstring injury to Rasmus Hojlund means he could make his debut in today's Community Shield showdown with Manchester City.
'He will have to make a step up and we will have to help him,' Ten Hag said.
Joshua Zirkee will have to make a 'step up' after joining Man United, Erik ten Hag has warned
'It's not like when you have a proper pre-season with the team. He went to the Euros so had to train on his own. We will need time to fit him in and that will take some weeks.'
Zirkzee scored 12 goals in all competitions in 2023-24 and has already had three seasons of top-level football wth Anderlecht and Bologna.
His experience includes European fixtures and a couple of outings at Euro 2024, coming on as a sub as the Netherlands lost 2-1 to England in the semi-finals.
He has won the Bundesliga with Bayern Munich in 2019-20, scoring four goals in nine games as a bit-part player.
'He is really hungry to do this because he is looking forward and he is also very excited to be coming into the Premier League,' Ten Hag added.
'To come to Manchester United and on his first occasion go to Wembley, of course that will highly motivate him, that's for sure.
'But he is also short in training, he had a break, he trained individually but he has now had a couple of days in the team training.'
At least the bedding-in process will be helped by familiarity with one character in Manchester.
Ten Hag also sought to play down the significance of the Community Shield result, whichever way it goes for his side.
'We know already we can beat City and that's not dependent on the result on Saturday, that will not change that belief,' he said. ' The FA Cup was a higher occasion than the game on Saturday.
'Nevertheless, we want to win, but we have shown we can beat City. We have that belief, but we don't want to take a risk with players and lose them for long periods.
'That's what we have to take from last season because when you get in a pattern when you're missing many players with injuries, that affects results. We want to get results and for that players have to be available.'