Mount is expected to be out for 'several weeks' with a leg injury after pulling up just 14 minutes into last weekend's Manchester derby.
The England international was making just his fourth Premier League start of the season at the Etihad Stadium and started just five games last term following his move from Chelsea in the summer of 2023.
In an emotional post on Instagram, Mount said: 'Words can't portray how devastated I'm feeling right now, you could probably see the look on my face when it happened. I knew what it meant.
'United fans, you may not know me too well yet, but one thing I can guarantee, I will never give up or lose faith.
'I've said this before, but I will continue to give everything, get through this tough period and won't stop until that's achieved.'
James was quick to post a message to his former Chelsea team-mate and said: 'Not many people understand what you're going through but I have lived your disappointment after disappointment after disappointment.
https://www.instagram.com/p/DDz_n1BtDsx/?img_index=1 'It's real tough and lonely man! Speedy recovery and stay strong brother! I have no doubts you'll be back to your best.'
Manchester United boss Ruben Amorim confirmed Mount's injury blow after his team's Carabao Cup exit at the hands of Tottenham.
'I don't know the exact date but it is going to be long,' he said.
Asked whether Mount is injury prone, the ex-Sporting Lisbon manager added: 'That is not my department.
'With Mason Mount, we are going to help him and it is really hard for him to be out so long. He is trying really hard. If they try hard, we will help them to the end.'
James was not alone in sending his support to Mount after his latest injury blow, with Manchester United captain Bruno Fernandes insisting the squad would be 'by his side' during his rehabilation.
'Always with you,' Fernandes said alongside a heart emoji. 'We will be by your side in every moment. We fall together, we rise together.'
'Mason Mount... I have to tell you I love that kid,' Amorim said shortly after his appointment at the Theatre of Dreams.
'You can look in his eyes that he wants this so bad. This is the most important thing.
'He played in this system, so it's perfect for him... I have two positions for Mason Mount, so he should be very happy!'
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