In his first season in charge, Ten Hag has ended a six-year trophy drought by winning the League Cup, has United in pole position for a return to the Champions League next season by sitting third in the Premier League and into a FA Cup semi-final against Brighton on Sunday.
Yet, a 3-0 defeat in the cauldron of the Ramon Sanchez Pizjuan on Thursday was also far from United's biggest or most humiliating under the Dutchman.
United have been smashed 4-0 by Brentford, 6-3 at Manchester City and 7-0 by Liverpool - matching the club's worst ever defeat - in a rollercoaster campaign.
"Everyone can see the demands and standard has to be higher in a club like Manchester United," said Ten Hag.
"It's unacceptable. We lost the battles, they had more passion, more desire, more willingness, that's difficult to win games."
A season of highs and lows for the fanbase is accentuated by the uncertainty surrounding the future of the club's ownership.
Supporters have long since wished for the departure of the Glazer family. But delight at the Americans announcement they were exploring a sale in November has turned to fear they could yet stay on by selling just a minority stake.
The delay in a solution to the sale process, with a third round of bidding to take place by the end of the month, risks leaving United behind when it comes to securing transfer targets this summer.
On the evidence in Seville, that is something the Red Devils can ill afford if Ten Hag's early impact is to not fizzle out into another false dawn in the club's decline since Alex Ferguson retired as manager in 2013.
Doubts grow over De Gea
Despite hundreds of millions spent in recent years, United's lack of depth without a few key players was exposed in Spain.
In the absence of injured first-choice centre-back pairing of Lisandro Martinez and Raphael Varane, United's defence was torn to shreds by a side that was fighting a relegation battle in La Liga a few weeks ago.
Ten Hag has left club captain Harry Maguire on the bench for most of the season and the England international showed why as he gifted Sevilla the perfect start with a misplaced pass that Youssef En-Nesyri capitalised on.
But Maguire was far from the only culprit as David De Gea's performance raised questions over whether United should shell out on a new contract for the Spanish goalkeeper.
De Gea's deal - which made him the world's highest paid 'keeper when he signed it in 2019 - is due to expire at the end of the season.
Reports suggest he will remain at Old Trafford with his shot-stopping capabilities winning Ten Hag's confidence.
But De Gea has always been a poor fit for Ten Hag's desire to play out from the back and a calamitous clearance handed En-Nesyri his second and Sevilla's third of the night.
By that point, the tie already appeared lost with United toothless in attack, once again exposing the need for a specialist number nine.
Marcus Rashford's career-best 28 goals have papered over the cracks, but the England international prefers playing off the left wing and was unable to turn around a sinking ship when he was thrown on at half-time on his return from a groin injury.
Bruno Fernandes' creativity was also missed with the Portuguese midfielder suspended.
"This shows the squad isn't good enough, if you have injuries," former United midfielder Paul Scholes told BT Sport. "This squad can handle other games but when it comes to the crunch, the big atmosphere, it falls short. The quality drops a level when the big players aren't fit."
Another expensive summer rebuild is required to get United back to the standards Ten Hag demands.
But whether he will have the resources he needs rests on who is running the club next season.
(For more sports news and updates, follow Ahram Online Sports on Twitter at @AO_Sports and on Facebook at AhramOnlineSports.)