In a statement, the club said that “the procedure has gone very well but he needs a period of intensive care to optimise his recovery”.

Sir Alex left Old Trafford in 2013 after 26 years, making him the club’s longest-serving manager.

In his time at the club he won 13 Premier League titles, two European Cups and five FA Cups.

The 76-year-old is now recovering from the emergency surgery at Salford Royal hospital.

Here The Independent looks at his condition.

What is a brain haemorrhage?

Put simply, it is bleeding in or around the brain. It causes swelling, and the pooled blood forms a mass known as a haematoma, increasing the pressure on the brain and reducing vital blood flow.

People who experience the condition will sometimes develop symptoms similar to a stroke, with weakness on one side of the body or a feeling of numbness.

Sometimes patients will experience a severe headache or difficulty speaking or seeing.

Sir Alex has reportedly suffered a variety known as a subarachnoid haemorrhage, which accounts for around one in every 20 strokes that take place.

What are the symptoms of this kind of haemorrhage?

Symptoms can include a sudden agonising headache – often described as similar to being hit on the head suddenly, as well as a stiff neck, feeling or being sick, sensitivity to light, blurred of double vision and loss of consciousness or convulsions.

What causes bleeding on the brain?

There are a number of reasons why brain haemorrhages can occur. According to the NHS, there are “usually no warning signs” to indicate a subarachnoid haemorrhage is about to take place, but it can be brought on by physical strain.

Luke Griggs, of the brain injury charity Headway, said the condition can sometimes happen spontaneously as a result of a ruptured aneurysm, otherwise known as a haemorrhagic stroke.

"A haemorrhage can also result from a blow to the head," he added, "often of a significant severity. But it can sometimes occur after a seemingly minor head injury.

Sir Alex Ferguson - A career in pictures Show all 15 1 /15 Sir Alex Ferguson - A career in pictures Sir Alex Ferguson - A career in pictures Becoming Manchester United manager Sir Alex was named Manchester United boss in 1986 following his success with Aberdeen Getty Images Sir Alex Ferguson - A career in pictures Saved by the cup After a rocky start, Ferguson was famously saved from the sack by winning the FA Cup in 1990 Getty Images Sir Alex Ferguson - A career in pictures Cup Winners' Cup The trophies kept coming from then on as he led United to the now defunct Cup Winners' Cup in 1991 Getty Images Sir Alex Ferguson - A career in pictures A first Premier League title... Ferguson led United to win the inaugural Premier League in 1993 Man Utd via Getty Images Sir Alex Ferguson - A career in pictures ... and a second... United retained the trophy and added the FA Cup and Charity Shield - Ferguson's first domestic double Man Utd via Getty Images Sir Alex Ferguson - A career in pictures ... and a third Another domestic double saw Ferguson reclaim the Premier League crown for Manchester United after losing out to Blackburn Rovers the previous season Getty Images Sir Alex Ferguson - A career in pictures A first Champions League Ferguson achieved his most memorable feat in 1999 winning the Premier League, the FA Cup and the European Cup Sir Alex Ferguson - A career in pictures Coaching the best Cristiano Ronaldo was just one of a number of mega-stars Ferguson coached Man Utd via Getty Images Sir Alex Ferguson - A career in pictures Another Champions League Nine years after his first, Ferguson claimed a second European crown, beating Chelsea on penalties in Moscow in 2008 Man Utd via Getty Images Sir Alex Ferguson - A career in pictures The Class of '92 Dubbed 'Fergies' Fledglings', Paul Scholes, Nicky Butt, David Beckham (all pictured), Gary Neville, Phil Neville and Ryan Giggs came through the academy to form the cornerstone of Ferguson's successful teams Man Utd via Getty Images Sir Alex Ferguson - A career in pictures The Sir Alex Ferguson Stand Manchester United renamed the North Stand after their legendary manager to mark his 25th anniversary in charge in 2011 Man Utd via Getty Images Sir Alex Ferguson - A career in pictures Immortalised in stone Outside the Sir Alex Ferguson Stand is the Scot's likeness in the form of a statue AFP/Getty Images Sir Alex Ferguson - A career in pictures A final Premier League In 2013, Sir Alex Ferguson won his 13th and final Premier League crown, retiring at the end of the season Getty Images Sir Alex Ferguson - A career in pictures Freedom of the city Ferguson was handed the freedom of the city of Manchester upon his retirement Man Utd via Getty Images Sir Alex Ferguson - A career in pictures The greatest Ferguson goes down as Britain's most decorated manager and considered the greatest who ever lived my many experts in the game Getty Images

"It can be extremely serious and require urgent medical intervention."

Other causes can include high blood pressure, leading to weakening arterial walls and causing them to rupture, or weakened blood vessels.

What does the surgery involve and how does it help?

In some cases of brain haemorrhage, an operation is needed to relieve the pressure on the brain. Draining the haematoma will reduce the size of the pooled blood, while surgeons can also repair damaged blood vessels.

There are several ways a surgeon can drain the pooled blood, including by removing a portion of the skull or drilling a small hole in it.

The decision to operate will depend on a range of factors, including the location of the haemorrhage.

A bleed on the brain can also be treated through medication to reduce the swelling.

What is the outlook for patients?

The outlook can vary depending on many factors, including where the bleed occurred and the size of it.

Mr Griggs said: "Every brain injury is unique, as is every individual's recovery."

Many patients will survive a bleed on the brain, but recovery can take many months. In some cases, extensive rehabilitation is needed to regain function, including speech therapy and physical therapy.

Some people can be left with persistent weakness or residual seizures, headaches or memory problems.

Since the news of Sir Alex’s condition occurred, tributes have flooded in from across the football world.

“I hope he is in good hands and I hope the operation is a major success,” said Everton manager Sam Allardyce, who described the news as “tragic”.

He added: “I hope he has a full recovery.”