The Highlanders play the Blues at Albany Stadium on Thursday afternoon in their last match before opening the Super 14 competition in 10 days' time.
Coach Glenn Moore said the actual state of Vainikolo's injury would be properly assessed today, after the swelling went down.
Vainikolo damaged his left hamstring about 10min into Friday night's match against the Crusaders.
Vainikolo had strained the same hamstring during the Christmas break but had made a steady recovery, under the supervision of medical staff, and last week was his first start.
But he was injured while making his first run and left the field immediately.
The Tongan flier was jogging lightly at training yesterday but Moore said he would probably not play against the Blues.
Vainikolo had an outstanding season for the Highlanders last year, and his unavailability for any length of the season would be a severe blow for the southern franchise.
Another winger, Lucky Mulipola, suffered a mild foot sprain and also received a bang to the knee in Friday's game, but he had recovered from the injuries and was able to train yesterday.
Lock Ross Kennedy has a calf strain and sat out training.
Kennedy has a history of calf trouble, and Moore said it was unlikely Kennedy would train today and would have the injury assessed.
With fellow lock Tom Donnelly out for at least a month after rupturing ligaments in his thumb, locking ranks are already thin for the franchise, but Moore said no locking replacement would be called in.
All other players are injury-free.
Moore said he had studied the 40-17 loss to the Crusaders and said the side spent a lot of time defending. He said all the damage happened between the 80th and 90th minutes of the game, played in three 30min periods.
The side at times was too slow to defend counter-attacking and not spreading out enough.
The Crusaders scored three tries in the last 7min of the game.
"But I'm still reasonably happy with what we saw. In the first 30 we had phases when we had our game plan going and asked questions of them," he said.
