Undoubtedly Mickelson will be the subject of considerable conjecture when the golf world returns to Pebble Beach in June for the 119th playing of the U.S. Open, the major that defines Lefty's bouts of futility more than anything. "I'll deal with that in six months".
Mickelson had wanted to finish in the dark on Sunday and was left frustrated when Casey refused to play on - but a day later he understood the Englishman's stand.
Casey missed an eight foot birdie chance on the penultimate hole while his long eagle attempt at the par-5 18 ran six feet past and after watching Casey make his closing birdie to secure outright second, Mickelson made no mistake from a similar distance to close out a -7 65 to complete a three-shot victory.
Expanding on why he felt his performance at Pebble Beach boded well for Augusta, the three-time Masters victor said: "The two areas that you have to be at your best to play well at Augusta are you've got to hit the ball long and have a really good short game and those were two areas that were very helpful here".
Just as he and Casey were about to tee off for the start of their round on Sunday, severe weather hit the Monterey Peninsula, covering Pebble Beach in hailstones.
"The challenge is there are so many great young players in the game that it takes our best to win".
The popular lefthander has been a US Open regular since Irwin's victory nearly 29 years ago and he's finished second an incredible six times in the event.
Mickelson won on American soil for the first time since the Phoenix Open in 2013.
That also led him to enter this week's Genesis Open, which he had not planned to play, prior to defending a title at the WGC-Mexico Championship next week. All four players return to Pebble along with numerous game's best, including Jordan Spieth, Tony Finau and Jason Day.
"A phenomenal round of golf, " Casey said.
Joining Casey and Mickelson on the course Monday was Don Colleran, a FedEx executive who with Casey led the two-man Pro-Am competition by one stroke at 33-under-par.
After playing the 17th and 18th on Monday, Mickelson admitted Casey was right. "It's a totally different golf course. Sometimes I get in my little bubble and don't see the bigger picture".
But the final day, and all week, was about the ageless Mickelson. "We're going to have a good chance to come out on fresh greens".
He gained ground with birdies at the par-five second and the fourth, then made back-to-back birdies at the ninth and 10th - two of the toughest holes on the course.