Patience will be the key, says Casey.
Tournament officials from the United States Golf Association had hoped to eventually resume play at the Long Island, New York, course when play was initially suspended at 10.15am (3.15pm BST).
As the rain that swamped the greens and caused the opening round to be halted continued to pour into the afternoon, however, they decided no further play was possible and at 1.55pm declared play would resume at 7.30am today.
With rain expected to continue into today - and tomorrow promising a deluge equal to yesterday's - Mike Davis, the USGA senior director of rules and competitions, said a Monday finish was looking a distinct possibility as the body would not contemplate deciding the champion in fewer than 72 holes.
With the USGA also resisting calls to introduce a 'lift, clean and place' policy for muddied golf balls on fairways, world No 3 Casey, who will hope to restart today two shots off the four-way lead at one over par, said the eventual champion would need to be have plenty of patience.
"It's only going to get worse, but we will see what happens," Casey said of the weather forecast.
"You are certainly going to have to strike the ball very well, but it's going to be the guy who keeps his head this week.
"Patience is paramount and you are going to have to deal with not only a very tough golf course but also some of the decisions that are being made like not being able to lift, clean and place.
"That's going to be very difficult and you are going to have be patient and expect some difficult lies and some mud-balls.
That's the way it is." Asked if that reduced the tournament to a lottery, the Englishman replied: "It does, but hey, it all adds to the fun." Davis insisted the US Open would be allowed to run its natural course.
"The USGA is firm on this, that we will not determine a national champion until we play 72 holes," he said.
"So if that takes us in Monday or Tuesday, whatever." Of the golf that did get played before the suspension, defending champion Tiger Woods needed to be at his combative best to stay on track for back-to-back wins.
World N0 1 Woods was one over par after six holes alongside Padraig Harrington and Angel Cabrera when play was suspended with Americans Jeff Brehaut and Ryan Spears, Sweden's Johan Edfors and Canada's Andrew Parr sharing the lead at one under.
Woods was the only player to manage a finishing score under par when the par-70 Bethpage Black course first hosted the US Open in 2002 and it was already playing longer than its 7,426 yards as play got under way yesterday.
Having won his third US Open title at Torrey Pines last year, he got off to a bad start when he hooked his opening tee shot 40 yards left of the fairway.
Woods' second shot sent him into a greenside bunker, but he still managed to get up and down for par on the 430-yard dog-legged opener.
Woods came unstuck for the first time at the fifth, driving into the rough and finding a greenside bunker with his third shot.
Woods got to eight feet from there, but missed his bogey putt and had to settle for a double.
Woods battled back, sinking a birdie at the sixth from around 20 feet to get back to one over..
SWINGING IN THE RAIN: Course workers roll water off the 11th green at Bethpage State Park. Below, Paul Casey shelters under his umbrella