The pair did duty at left-back during the 2010 Soccer World Cup, but with Thwala recovering from knee and ankle injuries, and Masilela struggling with a knee problem, Mosimane has no experienced left-sided defender to call on ahead of the upcoming clash with Egypt.
South Africa face the African champions in a crucial 2012 Africa Cup of Nations qualifier in March and the result is likely to go a long way in settling the fate of Group G, which also includes Sierra Leone and Niger.
Although Mosimane is looking at several youngsters who could potentially fill the void, he bemoans the fact that the South African Football Association have failed to arrange friendly matches for him to experiment in.
"We have tried so many players on the left, such as Brett Evans, Innocent Mdledle and Bradley Carnell, and we have not found enough cover or depth," he told the Sunday Times.
"I think he [Nyiko Tshabangu] is a good player. I was also impressed by Tsweu Mokoro, Keegan Ritchie and Reuben Cloete but I am not happy because we don't have friendly matches to try them out.
"I asked for friendlies but it is not happening. We are only playing one match, against Burkina Faso next month, before the do-or-die game against Egypt in March."