Lyon's form has been fiercely debated over the past fortnight after he was treated harshly during a Sheffield Shield match at the Gabba last month.
However, Clarke, who seems to have seen enough of Lyon in the nets this week , said there was evidence to suggest that his confidence has not been dented.
"I faced him today and he bowled really well. He is mixing his pace up really well and he has nice shape on the ball," News.com.au quoted Clarke, as saying.
"I'm confident that Lyon will be fine if he's the opportunity to play on Friday. In the past he's made the most of opportunities for Australia and bowled really well," he added.
Asked whether he would consider playing a pace quartet against the South African team, Clarke responded, "Ask me that in two days. I don't know the 11 right now."
"It is hard to tell now because there are grass clipping on top which make it appear whiter but it is green underneath. If its a green-top you have to think about four quicks, but I do like to have a spinner in the team," he added. (ANI)