The injury kept him out of the 71-31 win over the Sharks on Saturday and he will be assessed over the next few days.
England’s rookie fly-half Marcus Smith has been called up by the Lions as cover and will join the squad in Cape Town on Sunday.
With the Test series against South Africa due to begin in a fortnight the timing of the injury is horrendous for Russell.
“Finn’s had an injection. It’s not a strained Achilles, there’s a slight, wee tear in it. That’ll keep him out for the next couple of weeks,” said Warren Gatland the Lions head coach.
“We’ll see if we can rehab him and he’s going to be in a boot for the next five days. When he comes out of that, he’ll have some more treatment over the next five days.
“If he makes good progress we’ll assess him going forward. If he’s not, then there’s a possibility he may go home. It’s a wait and see situation with him.”
Russell had been named as a replacement for the Sharks game but had to step down shortly before kick-off.
The Racing 92 stand-off impressed in the tour opener against Sigma Lions last weekend and also showed nice touches after coming off the bench in the win over Sharks in Johannesburg in midweek. The injury will come as a real blow to the player who is a contender for the Test team.
“We’re obviously disappointed for Finn, who has been outstanding since he came into camp in Jersey, but we are optimistic he’ll still play an important role in the Tour,” added Gatland.
“We have two experienced fly-halves in Owen Farrell and Dan Biggar, so it’s a great opportunity for the continued development of Marcus.
“I’ve been following him closely for the past 12 months and I’ve been impressed with how well he’s matured as a player.
“Finn and Marcus are similar in that they like to play with a bit of freedom so, for me, it’s as close to a like-for-like cover as we can get. We look forward to welcoming him to the group in Cape Town.”
Smith played in England’s 70-14 rout of Canada on Saturday having only made his England debut against the USA last Sunday.
A week before winning his first cap, the 22-year-old playmaker lifted the Gallagher Premiership crown with Harlequins.
The Lions are due to play next on Wednesday but it is unclear whether the match will be against the South Africa A team or the Stormers. The tourists will then play again next Saturday against the team they didn’t play on Wednesday
The opening Test against the Springboks is on July 24 in Cape Town.