Former British and Irish Lion Matt Dawson says "the wheels are off the tour" if the side records a second loss against the Stormers on Saturday.
They face provincial side Stormers at the same venue in their last outing before the three-Test series against the Springboks begins on 24 July.
"The Lions have got to bounce back," Dawson said on BBC Radio 5 Live.
"If the Lions lose this game, the wheels are off the tour. They'll lose the series. There will be that siege mentality among the players.
"They [those playing on Saturday] have the responsibility now to give the momentum back to the Lions. I expect the Lions to be full ball."
'You have to be cute and clever, not outpower Springboks'
Although some labelled Wednesday's game as a "fourth Test", the Springboks had a stunted build-up to the match with their win against Georgia on 2 July the side's only official international since winning the World Cup in 2019.
After a second game against the same opponents was cancelled because of coronavirus cases in both camps, South Africa's A team will get another run-out against the Bulls at Cape Town Stadium immediately before the Lions' game on Saturday.
Head coach Jacques Nienaber has made wholesale changes to his team, with only Damian de Allende, Marco van Staden and Joseph Dweba keeping their places, although the squad still boasts nine World Cup winners.
Despite South Africa's lack of game time and recent six-day isolation because of a Covid-19 outbreak, the hosts stifled the Lions' attack and only allowed their opponents over the tryline once.
"First and foremost you have got to win the physical battle," advised Dawson.
"Boil it down to the basics, the Lions weren't up to speed. How were South Africa up to speed when they have hardly played any rugby?
"The Lions do have to look at themselves and I think Warren Gatland will be the perfect coach to bring that out of them.
"If you think you're going to go and outpower South Africa, I think you're barking up the wrong tree. I think you've got to be cute and clever."
Captain Jones 'has to start' - Shanklin
Lions attack coach Gregor Townsend - whose coronavirus isolation ended earlier this week - says the Stormers game is "massive for everyone involved".
Gatland said on Thursday his Test selection was still "fluid" and Townsend added that coaches would begin discussing their team for the series on Sunday.
The Scotland head coach described the Stormers game as "the last chance for players to put their hands up".
For returning captain Alun Wyn Jones and rookie fly-half Marcus Smith, it is also their first.
Jones joined up with the Lions on Thursday after an incredible 18-day recovery from a dislocated shoulder, while Smith was called up last Saturday as cover for Finn Russell, whose Achilles tear is likely to keep him out until the second Test.
Dawson describes the return of four-time Lion Jones as "gold dust" and former Wales team-mate Tom Shanklin backed the lock - who is on the bench on Saturday - to be ready for the Tests.
"If he wasn't fit, he wouldn't jeopardise the tour," Shanklin said.
"He has to start. There is no way he can come off the bench because if he comes off the bench and injures himself then they're two men down straight away.
"He's not there to give some motivational speech. He's out there because he's fit."
'Biggar and Farrell can learn from Smith' - Monye
The inclusion of 22-year-old Smith is the culmination of a remarkable three weeks that have included winning a Premiership title with Harlequins and earning his first England cap.
Although a place in the Test side seems unlikely, he and Owen Farrell are the only fully fit 10s available to Gatland with Dan Biggar rested because of a minor ankle injury.
Former Quins wing Ugo Monye said Smith will offer "something magical" when he makes his first Lions start on Saturday.
"I think he brings an absolute vitality and freshness, instinct rugby," added the former Lion.
"I hope he retains his personality and style of play and we get to see that on Saturday. I think this will be the easiest game he has played in the last three weeks.
"The opposition isn't as good and the quality of players he has around him is unbelievable.
"You have got a 22-year-old on tour with two caps and I honestly think Dan Biggar and Owen Farrell can learn from him."