After snatching the 200m backstroke world record in March, McKeown has been in ominous form in the lead-up to next month's world championships in Fukuoka.
She fell 0.05 seconds short of her 100m backstroke world record (57.45) at world championship trials in Melbourne on Wednesday, a day after a dominant win in the 200m individual medley.
Welsh, who won a backstroke title at the 2001 world championships, was impressed by McKeown at the Melbourne Sports and Aquatic Centre, seeing a rising talent who could one day be counted among the greats of Australian swimming.
"It's absolutely incredible. She's 21, she's got so much more to give," Welsh told Reuters.
"She's mentally and physically strong. Just walking on the pool deck you can see the strength in shoulders and arms.
"But in the race, she stays calm. She's able to pace things really well.
"We haven't seen the best from her yet but we've seen incredible things already."
Olympic 100m and 200m backstroke champion McKeown has also shot into medal contention for the 200m individual medley (IM) after being pipped to gold in the event by American Alexandra Walsh at the world championships in Budapest last year.
McKeown's versatility means she could also help Australia's bid for the relay medley title in Japan by swimming the breaststroke leg rather than her favoured backstroke.
Australia's swim team will use Fukuoka as a yardstick to measure progress in the lead-up to next year's Paris Olympics.
Expectations are high after the team took an Australian record 21 medals, including nine gold, at Tokyo, second only to the United States' haul of 30 medals.
Teenage talents have made their mark at the world championship trials this week, including 19-year-old Mollie O'Callaghan, who upset Olympic champion Ariarne Titmus to win the 200m freestyle on Thursday with a year's best time of one minute, 53.83 seconds.
Sam Short, also impressed with a year-best 7:40.39 to win the men's 800m freestyle and qualify for Fukuoka.
The emergence of new young guns bodes well for Australia's hopes of continuing the gold rush in Paris, said Welsh.
"Depth has usually been our challenge, especially compared to the United States, but we have some amazing peaks (of performance) in our stable of swimmers," he said.
"We're doing pretty well with what we've got at this stage."