According to Liam Prenderville in the Mirror, United's share price has dropped from $19.58 in July 2014—soon after Van Gaal was announced as David Moyes' successor—to $13.33 on the close of the New York Stock Exchange on March 23, 2016.
With the Dutchman's first game in charge having been a 7-0 friendly win over LA Galaxy on July 24, 2014, it means "over $1 billion has been wiped off the club's share price value" since.
The report added that analysts blamed huge spending on players and poor performances on the pitch for the drop in value. Laith Khalaf, a market analyst for Hargreaves Landsdown, gave further explanation for United's financial downturn, per Prenderville:
Football clubs typically don't present a particularly attractive investment proposition because their revenues are lumpy, and their employees are rather expensive.
Mistakes on the pitch can be costly for shareholders too—in 2015 Manchester United’s broadcast revenues dropped by £30 million as it failed to qualify for the Champions League in that season.
So far this year things seem to be picking up with a bounce in revenues, but if the club fails to qualify for the Champions League this season, next year could be a fallow one for investors.
The blame for not qualifying for the 2014-15 UEFA Champions League cannot be placed at Van Gaal's door, his predecessor Moyes having been responsible for the seventh-placed Premier League finish in 2013-14, the season after Sir Alex Ferguson had led United to the title.
Indeed, the former Ajax, Barcelona and Bayern Munich boss was appointed to get United back into Europe's top club competition, a target he duly met with a fourth-placed Premier League finish last season.
However, the Red Devils were embarrassingly knocked out of a relatively easy group in the 2015-16 Champions League and have seemingly regressed under Van Gaal this season.
They are desperately fighting for a top-four finish, but they sit in sixth, one point off the fourth spot held by local rivals Manchester City.
Another season out of the Champions League and United's value would likely take another hit.
It could also potentially see Van Gaal lose his job despite his contract extending into 2017, with Jose Mourinho seen as the front-runner to take over at Old Trafford, per Spanish newspaper El Pais (via the Guardian's Ed Aarons).
Bleacher Report's Dean Jones believes the former Chelsea boss is the right man to revive the ailing Manchester club:
United have been on a downward spiral since Ferguson departed the club in 2013; Moyes' calamitous 10 months in charge was a dramatic comedown for the Red Devils and their fans following two decades of continued success.
The situation has hardly improved under Van Gaal, per the Manchester Evening News' Samuel Luckhurst:
The United hierarchy have been patient with the 64-year-old thus far, keeping faith with him even during a terrible run of eight games without a win at the end of last year.
However, the financial implications of United's relative failure under Van Gaal could finally be the deciding factor for his future.
With the powers that be at United unlikely to tolerate another season without top European football—not least because of the impact it will have on the club's revenue—a top-four Premier League finish looks like a must this season if Van Gaal is to keep his job.