Jose Mourinho and Man Utd fans will be grateful for the youngster as he came off the bench and scored a late winner against Hull City.
United dominated possession throughout, but struggled to find a way past a Hull defence determined to remain unbeaten at the start of the season.
The Red Devils' best chances of the first half fell to Ibrahimovic and Rooney. Firstly the Swede saw a header sail just over the bar and Rooney saw an effort cleared off the line by Curtis Davies.
Hull had chances of their own however, and Robert Snodgrass hit a free-kick agonisingly wide with De Gea beaten.
It was more of the same in the second half as United had the better of the possession and chances.
Ibrahimovic and Martial saw efforts fly just wide and United probably should have had a penalty when David Meyler appeared to block a cross with his arm.
However, once again, Hull chose their moments to attack and Tom Hudlestone saw his deflected effort go just wide.
United then introduced Rashford who looked lively from the moment he entered the field. The youngster forced Eldin Jakupovic into a good save with around 10 minutes to go.
Mkhitaryan also had an effort that was brilliantly blocked by Curtis Davies and Hull almost nicked the game late on.
Hull then almost stole it at the death when the ball fell to Meyler on the edge of the box, whose left-footed shot flew just over the bar.
Manchester United then made the most of Hull's inability to take their chances as Rooney broke down the left and fired a low cross into Rashford, who was in the right place at the right time to put the ball past Jakupovic.
It may be early days, but the current Manchester United side seems to have one of the great traits of the most successful of United sides - even when not at their best, they get results.
Spare a thought for Hull though, who defended heroically for 92 minutes - with 14 senior professionals in the squad - but conceded a gut-wrenching late winner.