The victim, who is in his 40s, sustained head injuries in the attack outside a licensed premises in Flax Street at about 05:25 BST.
He was taken to hospital and a police spokesman said he is "said to be in a critical but stable condition".
Det Insp Heather Whoriskey appealed to anyone who was in the area at the time to come forward with information.

Security forces were deployed on Monday morning and heavy gunfire was heard in the streets
The Ninjas were a major anti-government force in the 1997-99 civil war.
The violence comes weeks after Denis Sassou Nguesso won a third presidential term in a poll that the opposition said was marred by "massive fraud".
At least one police station and a government building were attacked in the Makelekele district.
The Ninjas were loyal to former Prime Minister Bernard Kolelas, the father of Guy-Brice Parfait Kolelas, who stood in the presidential election in March and got 15% of the vote.
Mr Sassou Nguesso, in power for more than 30 years, won the election with 60% of the vote.
Reuters news agency reports that on Monday morning young opposition supporters were chanting "Sassou, leave!" in Makelekele , which is in the south of the capital city.

The Germanwings plane carrying 150 people crashed on its way from Barcelona to Duesseldorf.
Mr Hammond said "based on the information available to us" it was likely British nationals were involved.
But he said he would not speculate on numbers of Britons involved until checks were completed.
"We are working closely with the French, German and Spanish authorities, and the airline, to establish the facts," he said.
He also said the UK Air Accident Investigation Branch, and UK disaster victim identification experts, were standing by to offer assistance to the French authorities, if required.
One passenger on the plane was named as Marina Bandres Lopez-Belio - a UK resident originally from Jaca in Spain - by the mayor of her Spanish home town.
The mayor of Jaca, Victor Barrio, said Ms Bandres was on the flight with her baby son Julian and had been in the Spanish Pyrenees town to attend a relative's funeral.
But he said he did not know if her husband was also on the flight with her.
Ms Bandres's former school, Colegio Escuelas PÃ­as in Jaca, issued a message of "sincere condolences" to her family.
The Manchester Evening News reported that she had lived in the Fallowfield area of Manchester and Rochdale since arriving in the UK.
The Airbus A320 - flight 4U 9525 - went down between Digne and Barcelonnette.
There are no survivors, officials say.
France's interior minister said the "black box" flight recorder has been found but the cause of the crash is not yet known and the plane sent no distress signal during an eight-minute descent.

Investors were expected to remain cautious until the Fed announces its decision on Thursday.
The Dow Jones rose 140.1 points to 16,739.95, while the S&P 500 added 17.22 points to 1,995.31. The Nasdaq was up 28.72 points at 4,889.24.
Figures released on Wednesday showed consumer prices fell 0.1% in August.
Prices were up 0.2% from a year earlier, indicating that inflationary pressures within the US economy remain weak.
Analysts think the Fed's decision is finely-balanced, as while inflation is weak, the economy is growing and the unemployment rate has fallen to 5.1%.
"You can make a strong case either way for the Fed to begin raising interest rates or waiting," Ryan Sweet, a senior economist at Moody's Analytics, told Reuters.
"The prudent risk management approach would argue for them to hold off, but if the Fed was really data dependent there is a very strong case to raise rates on Thursday."
Among individual stocks, Anheuser-Busch InBev rose 6.8% after it said it had made a takeover approach for rival brewing firm SABMiller.
FedEx shares fell 2.8% after the delivery company cut its full-year earnings forecast.
The company predicted earnings per share of $10.40-$10.90 for the year to 31 May, down from a previous forecast of $10.60-$11.10.
A man is critically ill after he was attacked by a number of people in an overnight assault in north Belfast.
[NEXT_CONCEPT]
The Congolese government has blamed attacks on government buildings in the capital, Brazzaville, on the Ninja militia group.
[NEXT_CONCEPT]
Some British nationals were "sadly likely" to have been on board the plane that crashed in the French Alps, Foreign Secretary Philip Hammond says.
[NEXT_CONCEPT]
(Close): US markets closed higher as the Federal Reserve begins its two-day meeting which will discuss whether to raise interest rates.