Manchester United's high wire act this season - stumbling while flirting with danger before somehow surviving - finally ended with a painful fall as Arsenal condemned them to two seasons without success for the first time in 26 years.

United's obvious frailties have been covered by the results that took them to the FA Cup quarter-final against Arsenal and also into the top four in the Premier League.

Victory would have allowed manager Louis van Gaal to put an optimistic gloss on the closing phase of a mixed season. Instead, Monday's 2-1 defeat raised serious questions of just how far United have come since the sacking of David Moyes and the appointment of the highly-regarded Dutchman.

Five games to save United's season

Van Gaal made the FA Cup a prime trophy target in his first season with no European football as a distraction and the Capital One Cup pushed out of reach by a humiliating 4-0 thrashing at MK Dons.

Now he must make sure the not insignificant prize of a place in the top four and a return to the Champions League, United's natural habitat under Sir Alex Ferguson, is secured.

When United beat Liverpool 3-0 at Old Trafford on 14 December, they established a 10-point lead over Brendan Rodgers' side, who were languishing in 10th place and off the pace.

Since then, Liverpool have been rejuvenated spectacularly and now - along with Arsenal and Tottenham - pose a serious threat to Van Gaal's remaining ambition for this season as they stand only two points adrift.

Quarter-final match facts Manchester United midfielder Marouane Fellaini conceded eight fouls during the game - the entire Arsenal team managed only 11 Manchester United captain Wayne Rooney has scored eight goals in his last nine FA Cup matches There have now been five red cards in the last eight FA Cup matches between Arsenal and Manchester United Danny Welbeck has scored eight goals for Arsenal since leaving Manchester United last summer - he scored 29 for United

How United cope with their next five Premier League games will surely shape the outcome.

They face sixth-placed Spurs at Old Trafford on Sunday (16:00 GMT) with Mauricio Pochettino's side three points behind United. Then there is a pivotal game at Anfield on Sunday, 22 March. On current form, Liverpool will be favourites.

This is followed by two home games, firstly against struggling Aston Villa before the derby with Manchester City on Sunday, 12 April. A visit to Chelsea completes this potentially defining period.

When it is over, it is likely Van Gaal will be closer to knowing whether his target has been achieved. If it has, it will shape the mood of Manchester United's summer.

In Van Gaal's defence, it would have been unrealistic to expect him to finish above Chelsea and Manchester City, given their level of investment and his own need to rebuild - but missing out on the Champions League now would be a devastating blow.

Former captain Roy Keane said it would be a "disaster" if United did not make the top four. The Champions League is not just lucrative, it is the biggest lever to attract stars of the stature they will need. Van Gaal's future prospects would be bleaker should he fail.

Transfer failures

The sight of Danny Welbeck, the striker Van Gaal let go, striking the decisive blow to send Arsenal into the FA Cup semi-finals while Radamel Falcao was not even summoned from the bench as United's saviour, was heavy in symbolism.

Van Gaal's cold explanation of Welbeck's sale to Arsenal for £16m on transfer deadline day in September was that "he was more a substitute than a line-up player".

On a night that further stalled United's progress under the Dutchman, it was the celebrated Falcao who was the substitute while Welbeck was not merely a line-up player but also Arsenal's match-winner.

If ever there was an evening to underscore the fact that Old Trafford is still waiting for Van Gaal's new names to sparkle, this was it.

And there was no bigger example than £59.7m British record signing Angel Di Maria. The Argentine, for once stationed in his correct position on the flank, did show flashes of his gifts, particularly with a perfect cross for Wayne Rooney's equaliser.

His night ended in ignominy, however, when he foolishly dragged at referee Michael Oliver's shirt after being booked for diving, resulting in an inevitable second yellow.

It was an inexcusable piece of blockheadedness - but perhaps also hinted at Di Maria's mental turmoil at his current failure to transfer the brilliance he showed for Real Madrid and Argentina to Old Trafford.

Luke Shaw, signed from Southampton for £27m, suffered another injury, while the suspicion lurks that Ander Herrera has not convinced Van Gaal. He was taken off at half-time. Moyes almost signed the Spaniard and he pitched up again after Van Gaal arrived. How influential was the current manager in that deal?

Midfielder Daley Blind was pedestrian and has rarely looked like a game-changer, while defender Marcos Rojo was rash and unconvincing in attempting to cope with sharp interchanges from Arsenal's attackers.

Angel Di Maria has no excuse says Manchester United manager Louis van Gaal

And so, back to Falcao. The Colombia striker was a spectator, even when United needed a goal to save their hopes of a trophy. He was the stellar star supposedly destined to add that gloss and ruthless finishing it was felt Welbeck would never bring.

It is hard to see how United can justify completing what would be a hugely expensive deal for Falcao. These last few weeks must somehow be used to provide evidence he has not simply become an ornament, a vanity purchase by Van Gaal and United.

United paid £6m to sign him on a season-long loan from Monaco in September and would need to fork out £43.5m to make his deal permanent.

What is Van Gaal's philosophy?

Van Gaal rightly deserves instant respect for a decorated career - but the sight of Marouane Fellaini and Chris Smalling up front in the closing stages and target for a succession of long balls was hardly the stuff of a tactical innovator.

Indeed, it smacked of tactical bankruptcy rather than the "philosophy" Van Gaal has espoused since his arrival. You can guarantee Moyes would have been pilloried for a similarly stone-age approach in a losing cause.

The assumption is that a coach of Van Gaal's calibre has a clear plan. The problem is that on many occasions this season it has been tough to detect what the plan actually is.

Louis van Gaal has won cup competitions in the Netherlands, Germany and Spain

Fellaini has improved after a dreadful first season following his £27.5m move from Everton but his role appears to be as United's blunt instrument in times of trouble.

The Belgian represents a lumbering legacy of the failed Moyes era, an uncomfortable fit with United's finest traditions. He was booked for persistent fouling on Monday and, while not their worst performer, he is hardly a signpost for a bright new generation.

Van Gaal still shifts between three and four at the back in a defence riddled with the sort of uncertainty that has Smalling conceding possession on an alarming basis and Antonio Valencia setting up Welbeck's winner.

If it was not for the world-class goalkeeper David De Gea, United's rivals for a place in the top four would be scenting more blood than they already do because defensively the 20-time English champions are often shambolic.

De Gea's brilliance has covered up gaping cracks. Phil Jones and Smalling have not kicked on, indeed have gone backwards, while Jonny Evans is suspended and Rojo has been mixed.

At least Rooney has been restored as a striker and responded with another goal but after a promising first half, Van Gaal's side were sloppy, slow and second best after the break. Arsenal were deserved winners.

With Robin van Persie a fading force and Falcao and Di Maria under-achieving, an old truth remains. United are still so reliant on Rooney. For drive. For goals. For determination. For all the names added to the squad, England and United's captain still remains the most important.

Van Gaal, whose team have won one more game than they had at this stage last season under Moyes, will apparently be given another £100m for further adjustments in the summer. Perhaps this will help make a philosophy which currently looks muddled and lost in a fog come into clearer view.

United's ill-discipline was also revelatory. Di Maria's red card was madness, his tug on the shirt of referee Oliver - who gave a magnificent display of officialdom under the extreme pressure of a home crowd and players in an increasingly blind panic - coming after a dive.

Adnan Januzaj also attempted to deceive Oliver with a dive so late it almost came in the opening moments of Sunday's game with Spurs. Desperation did not cover it.

They also picked up seven bookings, a guarantee of FA censure. So, you get the idea - this was a dreadful night for Van Gaal and Manchester United.

Success will come with fourth place. This alone is a measure of how far they must travel to even dream of revisiting any of the glories of the Sir Alex Ferguson years.

Another year for Arsene Wenger

Arsenal manager Arsene Wenger was under pressure and scrutiny after a display that mixed gross incompetence with naivety when they lost 3-1 at home to Monaco in the first leg of their last-16 Champions League tie.

And yet here at Old Trafford, scene of so many recent disappointments and a place where they had not won since 2006, the FA Cup holders confirmed a return to Wembley with an accomplished performance that mixed Wenger's traditional passing demands with a touch of steel.

As they had done when winning at league champions Manchester City earlier this season, Arsenal showed a mental block that had plagued them for seasons had been cleared.

Then, to add further pleasure, they drew either Reading or Bradford City in the semi-final. No easy tie - remember how Bradford unseated them on penalties in the 2012 League Cup quarter-final - but self-evidently the opponents the remaining sides would have wanted.

Arsenal are now overwhelming favourites to reach a second successive FA Cup final and the opportunity to retain the trophy won from two goals down against Hull City in May. Their season is likely to be alive until the last day with the possibility of another trophy.

And that, given Arsenal's faith in Wenger, is likely to be a shield against the critics who question (with justification it should be said) the lack of title challenges and struggles to reach the later stages of the Champions League.