The Tongan-born winger was trapped at the bottom of a ruck midway through the second half and had to be substituted as a precaution. Having been assessed at Watford General Hospital after the match, Vainikolo underwent follow-up scans this morning at The Princess Grace Hospital in London under the care of consultant Peter Hamlyn.
"The scans have confirmed there is no damage to the bones of the neck, though Lesley has suffered some bruising to the muscles in that area," said Hamlyn.
He will be monitored for the next couple of days and will return home on Thursday.
Gloucester's head coach Dean Ryan added: "He has made a fantastic start to his union career and we will obviously miss his contribution on and off the field but it will allow other members of the squad to push for selection.
"We are naturally extremely relieved that the injury is not more serious."
Vainikolo, who joined Gloucester from Bradford Bulls this summer, scored five tries against Leeds on his Guinness Peremiership debut two weeks ago.