Marco Van Staden added to training squad as Springboks sweat over Siya Kolisi fitness

Marco van Staden, pictured playing for the Bulls against the Blues in Auckland, has been drafted in to supply injury cover for Springboks captain Siya Kolisi. MICHAEL BRADLEY/AFP/Getty Images

Bulls loose forward Marco van Staden has been added to South Africa's training squad on Monday as doubts remain over the fitness of Springboks captain Siya Kolisi ahead of the Rugby Championship.

Kolisi sustained a knee injury playing for the Stormers in Super Rugby, and he was initially ruled out for six weeks in late May.

He now seems likely to need a longer period of rehabilitation.

"Van Staden has been drafted in as cover for Siya Kolisi and will remain in camp until there is absolute clarity about whether Kolisi will be able to take part in the initial stages of the Rugby Championship," SA Rugby said in a statement.

"Kolisi is continuing with his rehabilitation in camp."

Van Staden, 23, has two Test caps, earned in home wins over Argentina and Australia last year.

The Springboks host Australia in the tournament opener in Johannesburg on July 20 before travelling to Wellington to face New Zealand a week later.

Their final fixture in a shortened Rugby Championship season will be against Argentina in Salta in August 10.

The side will also host Argentina in a one-off test in Pretoria a week later before they head to Japan for the Rugby World Cup, which starts on September 20.

South Africa have been grouped with defending champions New Zealand, Italy, Namibia and Canada.