#1 To play in his favoured position and flourish

This is the biggest reason why Rashford needs to leave United, and it dates back to his arrival from the unknown to wunderkind status in 2015/16.
It seems obvious, but despite all his attributes, he's not playing in his favoured position - the striker, goalscorer, and effervescent talent that he's praised for being.
During his debut campaign - which was barely three months long - he scored eight and created two assists across all competitions. People were skeptical as to whether he was just fortunate or genuinely good, as is the case with most youngsters who breakthrough unexpectedly.
Crucially, he arrived when it mattered most and played a pivotal part in United's season. If it wasn't for his excellent solo strike at Upton Park against West Ham, who knows what would've happened to the Red Devils' successful FA Cup run?
If he didn't turn match-winner against Aston Villa, Arsenal, and local rivals City during the Manchester derby, United would have missed out on European football altogether as Southampton, West Ham, and Liverpool finished within six points of them at the end of the campaign.

Since then, his statistics in the final third (across all competitions) don't make for impressive viewing. In 2016/17, 11 goals and six assists in 52 appearances. This term, 12 goals and nine assists in 41 games - improving but even still, he should be aiming for much better numbers.
That starts with him playing in his preferred role again, as the centre-forward. Whether that be a lone striker or alongside someone else in a two-man partnership remains to be seen.
Despite having the creativity to succeed on the flanks, he's better suited elsewhere. It's increasingly unlikely that he'll regularly get that opportunity with Manchester United.