Batting first, the Australian tally of 227 should have been a lot better after they'd fought back to be 5-209 with almost 10 overs to play.
A late collapse, however, saw the tourists lose their last five wickets for just 18 runs in the space of six overs.
"We just didn't finish off our innings," said McKay.
"Put another 30 on our scoreboard and it's a different chase altogether. I think we can learn from that come Monday and hit the ground running."
McKay became just the fifth Australian to take a hat-trick in international one-day cricket - the first since Daniel Christian against Sri Lanka in Melbourne early last year.
The 30-year-old McKay is ranked seventh in the world one-day bowling rankings - the only Australian in the top 10.
He has a career average of 21.5 which places him inside the top 15 on the all-time international list.
It's no secret McKay also wants to play test cricket again, with his sole appearance coming in 2009 when he was a late call-up against the West Indies in Perth.
Given the injury problems in Australia's test fast bowling ranks it's certainly not out of the question and McKay agreed the scalps of England test batsmen Pietersen, Trott and Root in his second-over hat-trick could help that aim.
"It can't hurt, that's for sure," said McKay on Saturday night.
"[But] every game I get to play for Australia and Victoria back home is a chance to impress the selectors."
Australian opener Shaun Marsh suffered a right hamstring injury in Cardiff when trying to stop a boundary.
McKay said he'd be a "massive loss" if he couldn't start on Monday but players such as Phillip Hughes or Glenn Maxwell could step into the breach.
- AAP
