United slipped to a chaotic 3-2 defeat at home to Galatasaray on Tuesday and have lost their opening two Champions League matches for the first time.
It was United's sixth defeat in 10 matches in all competitions this season, with positive pre-season vibes now a distant memory.
Ten Hag appeared to be steering United in the right direction when he ended a six-year trophy drought last season and led them to a third-place finish in the Premier League.
Now the Dutchman, in his second season at Old Trafford, is under growing scrutiny for the first time as he scrambles for solutions.
Ten Hag's men badly needed a victory on Tuesday after losing their Champions League opener against Bayern Munich and faced a Turkish side who had never won in England before.
United led twice, courtesy of impressive goals from new forward Rasmus Hojlund (21), but Galatasaray took advantage of some calamitous defending, sealing the win when Mauro Icardi (30) dinked the ball over the error-prone Andre Onana (27).
Ten Hag says there are "no excuses" for his side's dismal performances but is not worried about losing his job.
"Last season: brilliant, terrific, more than we could expect," he said.
"We also knew in this project there would be common gaps. At this moment we are in a very difficult period as everyone can see but we come out together, we are fighting together."
The question is, what does the Dutchman, besieged by problems on and off the pitch, do to halt the steep decline?
Error-prone Onana
New goalkeeper Onana has become the symbol of the team's defensive woes but he is merely one malfunctioning unit in an underperforming machine.
The Cameroon international, who played under Ten Hag at Ajax, made a costly gaffe at Bayern last month and made another horrendous mistake on Tuesday as United imploded.
Onana was bought to replace the long-serving David de Gea (32), with his passing range considered one of his greatest strengths.
But his error count is mounting. On Tuesday, he passed to Dries Mertens (36) of Galatasaray. Casemiro's (31) attempt to rescue the situation but brought a red card and a penalty, which Icardi missed.
Ten Hag appears to be sticking with his number one, signed in a deal worth up to £47 million, giving him a vote of confidence.
"We already have seen in games his great capabilities, also his personality after he made mistakes," he said. "He will bounce back."
United have conceded 18 goals in 10 matches this season, their highest tally after 10 games of a campaign since 1966/67.
In mitigation, they are grappling with a crippling injury crisis in defence - Diogo Dalot (24) is the only fit senior full-back and central defender Lisandro Martinez (25) has been ruled out for another extended spell.
Hojlund was the one bright spark on Tuesday, scoring a powerful header and putting United 2-1 ahead after an electric burst of pace.
But fellow striker Marcus Rashford (25) looks a pale shadow of the rampaging forward who scored 30 goals last season. He has managed one so far this term.
Ten Hag has also had to deal with off-field distractions including a spat with Jadon Sancho (23), who has been banished from the first-team squad.
Brazilian winger Antony (23) returned as a substitute against Galatasaray following a leave of absence to deal with allegations of domestic abuse against him.
Everything is taking place against a backdrop of continuing uncertainty over the ownership of the club, nearly a year after the unpopular Glazers said they were considering a sale.
Despite the appalling results, there is no evidence that Ten Hag's job is under immediate threat.
But ultimately he will know the buck stops with him as attention turns to Saturday's home match with Brentford.
