Manchester United's Champions League campaign got off to a hectic start as they fell 4-3 to Bayern Munich in Germany.

While the scoreline suggests a narrow affair, United struggled to keep up with Bayern and had a late double from Casemiro to thank for making the scoreline appear more favourable. The pressure has ramped up for manager Erik ten Hag.

Here are four things we learned from United's defeat.

Andre Onana has work to do

There's no point piling on Andre Onana. The goalkeeper has already admitted he was at fault for Bayern's opening goal and accepted he has not been good enough since making his move to United this summer.

It's an admirable stance but you can't ignore the facts. 14 goals conceded from six games which have seen several shaky moments is simply not good enough and fans may have to be patient before they see the best from Onana.

It brings back memories to David de Gea's early months at Old Trafford. The Spaniard was poor - arguably worse than Onana - but eventually found his footing and became one of the best goalkeepers on the planet.

United fans have been here before but it's time to persevere. De Gea's resurgence is an excellent example of what can happen if a goalkeeper is allowed time to settle in.

Midfield problems remain

One of the earliest issues identified this season was United's midfield imbalance. The signing of Mason Mount clearly didn't yield the early results Ten Hag was expecting but this performance made it clear that the former Chelsea man is not the only player at fault.

Casemiro may have scored twice but his late double only served to paper over the cracks of what was an otherwise frustrating showing from the 31-year-old, who simply couldn't match the athleticism on show in Bayern's midfield.

Next to him, Christian Eriksen's ageing legs stood out. The Dane is more of a playmaker than a defender but, with his time on the ball limited, Eriksen didn't get the chance to showcase his skills and instead struggled to keep up.

Ten Hag clearly thinks that Sofyan Amrabat is the man to fix United's midfield mess, and for the boss' sake, he better be right.

Mixed outing for Facundo Pellistri

With Antony and Jadon Sancho both away from the squad for various off-field reasons, Ten Hag listened to calls from fans and offered a chance to 21-year-old Facundo Pellistri.

Pellistri was handed some unnecessary pressure ahead of the game when Ten Hag claimed he was put in to exploit Alphonso Davies' perceived poor defending. Now under a more-intense spotlight, Pellistri had a mixed outing.

The Uruguay international was alright. He had some dangerous moments but his inexperience at the highest level was abundantly clear, and there's no guarantee he'll get another chance to prove himself in the coming weeks.

Ten Hag now has a real decision to make over his right winger. Alejandro Garnacho, who is more of a left forward, is the only real alternative in the squad, while Anthony Martial and Bruno Fernandes could both do a job out on the right.

Raphael Varane's return can't come soon enough

After the game, Ten Hag called on United's defenders to look at themselves in the mirror after giving away a number of sloppy goals.

The centre-back pairing of Victor Lindelof and Lisandro Martinez, who was clearly not up to match speed yet, lacked any sort of composure against Bayern and were crying out for an experienced head to restore some sense of calm.

Raphael Varane, who usually partners Martinez when fit, is out with a muscle injury but has made a cautious return to training ahead of a potential return against Burnley at the weekend.

For United's sake, Varane's return can't come quick enough.

READ THE LATEST MAN UTD NEWS, TRANSFER RUMOURS & GOSSIP

This article was originally published on 90min as Four takeaways from Man Utd's defeat to Bayern Munich.