But the closing weeks of a grueling campaign have proved beyond all reasonable doubt, in Marcus Rashford, United have found their future. The scorer of a stunning free-kick that won the first leg of this Europa League semi-final in Vigo last week, the teenager produced a sublime cross that allowed Marouane Fellaini to head in a goal last night which proved vital.
With Wayne Rooney’s United career winding down through its final perfunctory weeks and Zlatan Ibrahimovic, maybe, never to be seen in club colours again, Rashford’s late-season dominance presents Mourinho with a quandary.
Were Rooney the Rooney of three years ago and had Ibrahimovic not suffered that career-threatening injury, Mourinho would have been going into next season with the two veterans, plus Rashford and fellow youngster Anthony Martial, competing for the solitary forward position that is now the default setting of top clubs.
Instead, Mourinho is now certain to spend massively on a striker in the summer - probably French international Antoine Griezmann - which presents the potential for Rashford’s development to be hindered.
Of course, the ease with which United will be able to attract Griezmann and others will depend on whether Mourinho is offering Champions League football next season, an aim that moved a step closer as his side advanced nervously to the Europa League Final with Ajax.
Last night’s victory, hard-fought and unconvincing in parts like so many United performances this season, will lead to another 12 days of analysis of Mourinho’s first season in charge at Old Trafford, especially given the late sending off of Eric Bailly, who will miss the final.
It seems reasonable to believe only the most active Mourinho haters could deem a season that includes winning the League Cup, the Europa League and qualifying for the Champions League as anything other than an unmitigated success.
But should United lose in Stockholm and miss out on Europe’s top competition? That is where the debate will become far murkier.
Perhaps that is what Mourinho was referring to when, in the build-up to last night, he described the tie as one of the most important games in United’s history.
That seemed a tall claim, given United will now be appearing in their seventh major European finals. But, as far as Mourinho’s United career is concerned, last night was crucial - although not as crucial now as the final.
But, even if United should lose to Ajax, there have surely still been enough positives to emerge from this campaign to grade Mourinho with a solid B. Alex Ferguson famously said his aim at the start of every season was to end with a trophy - and, yes, “even” the League Cup counted among his targets, as he proved by winning it four times.
To that end, Mourinho has already passed the Ferguson litmus test and, while a plague of drawn games have cost United dearly, there is no doubt they are more difficult to beat than at any time since Fergie’s retirement, a quality that proved vital last night.
Nobody is claiming this is the greatest ever United team but Mourinho has already set a club record for unbeaten league games within the same season with 25.
For many clubs, periods of reconstruction feature long periods without silverware - a phenomenon with which Liverpool, for example, are fully acquainted.
If Mourinho can end this transitional season with two trophies - and a Europa League title that is the only major title United have never won - then for the first time in the four years since Ferguson’s retirement, United appear a club on the right track.