However, having scored just three goals in all competitions since his move to United, Sanchez, who British media say earns a club-record 600,000 pounds a week, has so far left those supporters underwhelmed.
Sanchez toiled in Saturday's final, creating double the amount of chances of anyone else in the match, while failing to create that killer opportunity, or have a shot on target.
"His performances, well they have to improve. They can't get any worse to be honest with you," former United midfielder Paul Scholes told BT Sport.
"I think the first few games of next season are vital to him, he needs to get fans believing they are going to get close to City. They need a big player."
Sanchez arrived at United having scored seven and assisted three goals in 19 league games for Arsenal in the season, on the back of his most prolific goalscoring campaign of his career in 2016-17.
His United career, though, is yet to take off. Sanchez has registered more than one shot on target in a league match just once for United in 12 starts, having done so eight times for Arsenal in 17 league matches.
There have been glimpses of what he can do -- two assists in United's 3-2 win at City and a goal and an assist against Tottenham Hotspur in the FA Cup semi-final -- but on the whole, Sanchez has remained largely ineffective.
Sanchez's only other goal contributions have come against Huddersfield Town, relegated Swansea City and League One side Yeovil Town in the FA Cup.
"Next season he will be more Alexis than he is now," United manager Jose Mourinho said last month.
Sanchez relies on his boundless energy, consistently completing more dribbles than any team mate. However, considering he turns 30 this year, his days as a dynamic forward are limited, making next season so important.
United will need to make an explosive start to unsettle comfortable champions City, and Sanchez has shown in the past he is capable of inspiring such. Now is the time to deliver for a new club and prove his worth.
($1 = 0.7421 pounds)
(Reporting by Peter Hall; Editing by Christian Radnedge)
