New Zealand Police said Ryder, 28, had been rushed to Christchurch Hospital after suffering the injuries early on Thursday morning and remained there in a critical condition.
"It appears that Jesse has been the victim of a serious assault and suffered head injuries as a result," Detective Senior Sergeant Brian Archer told a nationally televised news conference.
"Jesse was taken to hospital where he remains in intensive care in a critical condition in an induced coma after suffering multiple injuries."
Ryder reportedly suffered a fractured skull and a collapsed lung, though it is not yet known if the injuries are life-threatening.
Ryder, who had been drinking at the bar with team-mates from Wellington, had been involved in two incidents, the first outside a bar with three other people.
Two of the three then followed him across the road to a nearby fast food restaurant, where one of them apparently assaulted the cricketer.
Local media reports have stated that Ryder was hit from behind, though the police did not confirm this.
Police are examining closed-circuit video footage, and "at least 10 or more witnesses" apparently saw the attack, according to Archer.
"At this stage we have not identified the people involved in the incident but are following positive lines of inquiry," he added.
"We are asking for witnesses to come forward and to speak to us about it."
Ryder, one of the most gifted batsmen in New Zealand, was in Christchurch playing for Wellington against Canterbury in the semi-final of the domestic one-day competition on Wednesday.
He had been in a self-imposed exile from international cricket after a series of alcohol-related incidents, however.
Despite having publicly sworn off alcohol, Ryder had begun drinking again in recent weeks, New Zealand Cricket Players' Association chief executive Heath Mills said.
"We have been dealing with Jesse for 10 years and there have been numerous issues documented in that time," Mills said.
"Jesse has been fighting some demons over the last few years and he has struggled with them. It will be an ongoing struggle for him but we are doing all that we can to help him.
"It shouldn't be a surprise in the future if we have to deal with isolated issues."
In 2008, Ryder needed stitches in his hand after he punched a window in a Christchurch bar, an injury that kept him out of the game for several months.
He has also been in trouble for turning up to training still affected after a heavy drinking session and was reprimanded by governing body New Zealand Cricket last year after he and fellow New Zealand international Doug Bracewell were involved in a verbal altercation with bar patrons in Napier.
Following the Napier incident, Ryder voluntarily stood down from international selection to address his issues with alcohol.
"There are some alcohol protocols around the (Wellington) team but my understanding was that those protocols were being met so from that point of view the organisation does not have any direct concerns," Wellington chief executive Peter Clinton said.
"Our primary focus is his welfare. We remain very concerned and hope that he will recover."
Archer said he did not think that alcohol was a "contributing factor" in the altercation on Thursday.
Ryder had been expected to travel to India for the lucrative Twenty20 tournament with the Delhi Daredevils on Friday.
He was not expected to return to international cricket until New Zealand's tour of England in May-June, following their 0-0 draw in the recently completed home series.
An aggressive batsman with a superb eye and delicate touch, Ryder has made 1,269 runs in 18 Tests at an average of 40.93 with a highest score of 201 and 1,100 runs in 39 one-day internationals at 34.37.
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