Leg-spinner Tahir only took one wicket in the series opener, conceding 139 runs in the process, as the tourists' pace attack did most of the damage in a 153-run win.
There had been calls for the uncapped Dane Piedt to come into the side for the next match in Colombo, but Tahir has received Amla's backing.
"Imran is a quality performer," said Amla. "I am not concerned about him at all.
"He has been exceptional in the shorter format of the game. Test cricket is a challenging place.
"He will come good, it will take a few good performances to get his confidence back.
"He is a wonderful bowler. I'd say he is one the most highly skilled leg-spinners in the world."
South Africa simply need to avoid defeat in the second match, which begins on Thursday, to record a first Test series win in Sri Lanka since 1986.
But Amla urged his side not to get distracted by the prospect of making history.
"What is more on our minds is to play good cricket for the next five days," he added. "Fortunately we've got the backing of a 1-0 victory behind us.
"But the bigger picture is to play some solid cricket in this match. We come here with a clean slate, we have to forget what happened in the last Test."