Staffordshire Police said officers were called at 03:55 BST to the southbound carriageway, between junction 16 for Crewe/Stoke-on-Trent and 15 for Stoke-on-Trent.
The man was taken to hospital where he remains in a critical condition.
The southbound stretch was closed after the crash and reopened at about 10:00. Motorists were warned of delays during the morning.
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West Midlands Ambulance Service said the man, believed to be in his 20s, was lying on the hard shoulder when crews arrived.
Spokesman Jamie Arrowsmith said: "He had suffered multiple serious injuries and was suffering from a reduced level of consciousness.
"The man received specialist trauma care at the scene before being anaesthetised in order to stabilise his condition.
"He was then transported to Royal Stoke University Hospital on blue lights and sirens for further emergency treatment."

Gunners Joseph Rowbottom, 27, from Wrawby, North Lincolnshire, and Albert Venus, 22, from Hull, died on 24 May 1915 during the Second Battle of Ypres.
Their remains were found during work on a new housing estate.
Relatives attended Ypres Town Cemetery where their bodies were reinterred, in coffins draped with the union jack.
The soldiers' remains, with shoulder titles from the North and East Riding Batteries of the RFA, were uncovered in the town in April 2013.
Ministry of Defence (MoD) researchers were only able to identify the two men of the six found.
Gunner Venus's great nephew John Howden, 73, from Roos, East Yorkshire, was handed the folded union jack that had draped his coffin.
He said: "When they handed me the flag I had a lump in my throat and tears in my eyes and I did not think it would affect me that way.
"You are at a distance and all of a sudden you're involved and it's near, it's just emotional."
Mr Howden said he had not known about his great uncle until he got a letter from the MoD.
He said he was pleased the link was proved following a DNA test.
Caroline Rowbottom, Gunner Rowbottom's cousin's granddaughter, received the flag on behalf of his relations.
Mrs Rowbottom, from Brigg, North Lincolnshire, said it had been a shock when the family received an MoD letter.
"Up until that point we did not even know of his existence - we did not have him in the family tree, which is very sad.
"The ceremony itself was very moving and very well put together," she said.
The traditional five-gun salute did not take place due to the heightened security level in the country in the wake of last month's attacks in Brussels.
Gunner Rowbottom had been a milkman before the war and was nicknamed Milky Joe in the local newspapers.
Gunner Venus enlisted in the 2nd Northumbrian Brigade in December 1914 and joined the Hull Batteries to complete his training as a gunner.
A stretch of the M6 was closed after a pedestrian was hit by a lorry.
[NEXT_CONCEPT]
Six British soldiers who fought in the First World War have been given military burials more than a century after they were killed in Belgium.