As a result, Rashford barely saw first team minutes and had lost his starting spot in the squad under Rangnick, by the end of the season. He started just 13 games in the Premier League, often coming off the bench to no effect.
This season has been a completely different story, though.
United's excellent form this season has also coincided with a massive improvement in Rashford's displays as he is starting to vindicate Ten Hag's faith in him earlier in the season.
The England international is the club's top scorer with 14 in the league, miles ahead of Bruno Fernandes, who has five to his name this season.
Looking at the placement of his shots this season, he has been able to find the back of the net at the near post cutting in off the left, instead of trying to bend the ball around the keeper to the right, which he has done in previous seasons.
He is a lot more direct in his approach and finishing and it shows.
Rashford has shown striker-like instincts when inside the box this season, making himself available for shots. Of his 14 goals in the league, one has been with his left, ten with his right and three with his head.
Time and again, he has shown himself to be a handful physically even, backing himself to go shoulder-to-shoulder against the strongest of defenders.
His improvement is not only evident on the pitch, but the underlying numbers also show us that he is shooting a lot more and getting into dangerous positions in the box off the left flank - also playing centrally at times for United.
Looking at his heat map for this season shows a lot of activity on the left flank and from there, inverting into the box into the left half-spaces where he has been used to devastating effect.
In earlier seasons, he wasn't able to impose himself upon defences as he has done this season. His ability to score headers is an impressive weapon that he's added to his arsenal.
Rashford is now averaging a greater volume of shots per game, getting into dangerous positions more often and drastically improved as an offensive outlet for United - to the point where he is winning games for United all by himself through his individual brilliance.
Averaging about three shots per 95 (3.02), Rashford is consistently getting into good positions and testing goalkeepers without being too wasteful. His xG per 95 rate of 0.48 continues to grow, underlining his growth into a near world-class forward.
It's all coming together for the England international as Ten Hag reaps the benefits of a confident Rashford, who has always shown potential to be one of the most potent attackers in the league.
Whereas earlier he has been more useful as a counter-attacking weapon, Rashford has adjusted well to a more ball-dominant style of play under new management.
Whether his red-hot form continues will certainly have a big say in how well Manchester United finish their season, but for now, they're enjoying the renaissance of a man who simply cannot stop scoring.