Springboks coach Nienaber on new scrumhalf Cobus Reinach

South Africa will be without key flanker Pieter-Steph du Toit and scrumhalf Faf de Klerk for the deciding Test against the British & Irish Lions on Saturday as coach Jacques Nienaber was forced into making three changes to his team.

Franco Mostert moves from lock to the side of the scrum to replace Du Toit, who was injured inside the opening 20 minutes of Saturday's 27-9 victory that levelled the series at 1-1.

Lood de Jager, who made a massive impact off the bench in that fixture, takes over from Mostert in the second row.

Cobus Reinach has not featured in the series yet but replaces De Klerk, with Herschel Jantjies again on the bench despite coming on for De Klerk in the first two games.

Reinach's greater experience of having played in England likely got him the nod from the start.

South Africa have deviated from their preferred 6-2 split on the bench between forwards and backs and this time will have three backline players among the replacements, including flyhalf Morne Steyn who kicked the Boks to victory in the 2009 series against the Lions.

"This is a massive Test for us with the series on the line, so it was important for us to maintain consistency in selection following on last week's performance," Nienaber said in a statement.

"Cobus is an experienced player with a calm head and who can handle pressure and we believe he will be able to dictate play well alongside Handre Pollard.

"I am delighted for Morne - he is a world class player and his work ethic and positive attitude on and off the field since joining the team in (a training camp in) Bloemfontein has been impressive.

"He can also perform under extreme pressure, as he shown for the Springboks and the Bulls, and that skill will be invaluable in such a vital Test."

The tempestuous series has involved plenty of needle and Nienaber is expecting the same on Saturday.

"We are preparing for a physical encounter and we know that every small battle will count if we want to win the series," he said.

"This match is as important to us as it is for the Lions, so we know it is going to be tight and we will have to capitalise on every opportunity we have to score points and be effective in every area of our game."

Team: Willie le Roux, Cheslin Kolbe, Lukhanyo Am, Damian de Allende, Makazole Mapimpi, Handré Pollard, Cobus Reinach, Jasper Wiese, Franco Mostert, Siya Kolisi, Lood de Jager, Eben Etzebeth, Frans Malherbe, Bongi Mbonambi, Steven Kitshoff. Replacements: Malcolm Marx, Trevor Nyakane, Vincent Koch, Marco van Staden, Kwagga Smith, Herschel Jantjies, Morne Steyn, Damian Willemse.