Smith only made his England debut against the USA last Sunday and was helping Eddie Jones’ team topple Canada when news of his call-up by Warren Gatland was announced.
A week before winning his first cap, the 22-year-old playmaker lifted the Gallagher Premiership crown with Harlequins and had been instrumental in a swashbuckling passage to Twickenham.
Russell has been forced to withdraw from the bench for Saturday’s clash with the Sharks because of an achilles injury and he will miss the next three games of the tour, which include the opening Test against the Springboks on July 24.
“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,” Gatland said.
“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.”