Wellalage made his debut count on Tuesday night, chalking up Steve Smith as his first international wicket at a crucial point before Australia chased down the revised total of 282 inside 44 overs of the rain-affected match.
A dangerous left-arm spinner aged just 19, Wellalage skidded one through on Smith as he went back to pull on 53 just as the visitors looked to be getting in control.
But it was hardly the first time he has done damage to Australia.
The star of Sri Lanka's under-19s World Cup campaign earlier this year, Wellalage took figures of 5-28 to run through Australia in St Kitts in January.
Then with his team faltering with the bat, he produced a match-winning 52 at No.5 to ensure Sri Lanka would top their group in the junior World Cup.
Tuesday night's performance in Pallekele was not as dominant, but it was equally impressive given the stage.
Smith had made a clear plan to go after the 19-year-old, pulling a good-length ball to the square leg boundary as one of two fours in the spinner's second over.
"He's a very good cricketer from the beginning," Sri Lanka captain Dasun Shanaka said.
"I have seen him performing at under-19s level, so I think he will deliver in the next game and future matches as well."
Wellalage also got rid of Ashton Agar late to keep Sri Lanka in the match, before his night ended on a sour note.
Entrusted with the ball late, Glenn Maxwell took 11 off the 42nd over from the spinner to help Australia home moments later with nine balls and two wickets to spare.
"I'd never seen him before," Maxwell said.
"It feels like there is a new Sri Lanka spinner that comes out every time we play them ... There is always new footage you have to watch.
"He actually bowled pretty nicely. He was pretty tight and got a couple of wickets as well."
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