Sajed Choudry suffered a number of serious injuries during the incident near his home in Blackburn.
Metropolitan Police Assistant Commissioner Sir Steve House said passers-by should only help officers struggling with violent suspects "if they can safely do so".
The Mayor of London has warned it could take a generation to turn the tide of violent crime after four fatal stabbings in the capital in less than a week.
The Streatham MP was speaking at a Demos panel proposing how they can solve violent crime.
The youngster was found after emergency services were called to Belle Vale Road at around 9pm on Tuesday, Merseyside Police said.
The alleged victim was left in a serious but stable condition after suffering a number of stab wounds during an incident on Fairbridge Road, Islington on Sunday.
They killed 28-year-old Abdul Samad for his iPhone and little more than "aggression and blood lust" as they swept across west London on a scooter.
Five gang members who made drill music videos glorifying violence have been locked up after being caught with baseball bats and machetes on their way to attack rivals.
An 11-month-old boy remains in hospital in a critical but stable condition.
Sajid Javid also vowed to prioritise police spending as he offered an olive branch to rank-and-file officers following years of sniping over budget cuts and staffing reductions.
The police have reported several cases of violent crime across the capital after bank holiday weekend bloodshed.