Rory McIlroy, at 12/1, was considered to be Koepka's nearest rival.
Those odds didn't prove to be the best guide.
Koepka finished down the field in tied 30th, with Johnson only tied 24th.
Rahm will be hoping it's a different story when the PGA TOUR plays host once more in The Northern Trust this week.
This time, he's the one pencilled in as the favourite and a very clear one at that.
Those who had an each-way bet on Rahm two years ago were rewarded.
The Spaniard burst out of the blocks with a 64 and added rounds of 68, 69, 69 to finish in a tie for third. That was just two behind winner Patrick Reed.
It's fair to say Rahm has had a very different build-up this time.
In 2019, he'd finished tied seventh at the WGC-St. Jude Invitational just a fortnight earlier and tied 11th in the Open Championship at Royal Portrush the week before that.
This time he'll need to check his clubs for dust having not hit a ball in competition since he taking third place in the Open Championship at Royal St George's over a month ago.
In fact, Rahm has played just three times since that dramatic Saturday in early June when being informed he'd have to withdraw from the Memorial Tournament after a positive COVID test. He was six shots clear at the time; victory had been stolen from his grasp.
But the first of those three events proved a monumental one.
Rahm, like this week, went into the US Open as the 10/1 favourite and justified the billing by closing out a thrilling win to claim his first Major title.
While he didn't quite turn the extra swagger into wins in his subsequent two starts - tied seventh at the Scottish Open and tied third at Royal St George's - he played enough top quality golf in both events to justify his status as World No.1.
Worth a bet to win this week? It would be very hard to put anyone off - especially with that previous course form.
In an ideal world, Rahm would have a gold medal around his neck too. However, COVID messed with his plans again after he was a late withdrawal from the Olympics in Japan.
He elaborated on that WD, his liking for the course and the current state of his game when speaking to the press on Tuesday.
On his season so far
"It's been a great year, an unusual one with six Majors counting towards the FedEx Cup points, a lot of big events out here, and a little bit more competitive in that sense, more to play for, but it's been fun. I'm going to miss having so many Majors counted.
"It was weird, just the other day I was thinking, man, I just realised the Masters is extremely far away.
"I feel like I've been pretty consistent throughout my career, but playing really good golf this year, and to top it off with a first Major win has been great. So hopefully I can add in the next three weeks and become a FedEx Cup champion as well."
On Liberty National
"I have good memories from here. Last time I played really good golf, I believe I finished third and had a chance to finish as well. It's a great golf course. It's in impeccable shape. It's going to be a fun one.
"Hopefully, the weather doesn't get too bad, like it can in the Northeast. Hopefully we get a bit of a break and we can enjoy a perfect shape golf course."
On his position of 5th in the FedEx Cup standings
"The next two weeks, obviously the goal is to win, but the main goal is to get to the last event, to get to East Lake to the TOUR Championship in as high rank as possible.
"So got to keep the good play going. I'll take it week to week. I want to hopefully win those events as well, but the important one is the last one, so it's a bit kind of like a qualifying few rounds where you're just trying to get ready and get positioned for the TOUR Championship."
On pulling out of The Olympics
"This was a little harder to digest than Memorial because I've done everything the system tells me to do. I got all my negatives. First one on Thursday negative, Friday negative, Saturday I get my first positive. And then they tested me again that same day, and I apparently was positive again. Then I got tested the next two days. One was the saliva test, one was PCR, both negative. Got an antibody test done, so blood test, and I had the antibodies.
"So I can't really explain what in the world happened. I don't know if it's false positives or just what I had leftover from when I had COVID, just dead cells that were in there that we all know can happen. There's a reason why the PGA TOUR won't test you for a while after you've had COVID.
"So it's unfortunate, and I understand it's a weird case because I tested negative so quickly and tested negative and tested negative all throughout the UK, and I get here and the test is positive.
"It really is unfortunate. It sucked because I wanted to represent Spain. I wanted to play that one. I wanted to hopefully give Spain a medal. I was wishing for a gold medal, but just being part of that medal count for the country would have been huge. It was more devastating in that sense. I was more in the mindset of playing for them more than me.
"Still makes me a little sad, I'm not going to lie. I'm going to have to wait three more years hopefully to qualify for the Olympics, but I was really ready for this one."
On not playing a tournament in the last month
"I guess I could have tried to play Memphis (WGC-St. Jude), but I was already in the mindset of not playing it, so I stayed home.
"There's always a positive. I don't think I'll ever get a chance to have four weeks off in the middle of the summer and have some time to enjoy with my family. I did the first two weeks, I spent a lot of time with (son) Kepa because, with what we do, we're gone so often that I know I'm going to miss a lot of things, and I'm going to take advantage of times I get to be home, and that's what I did.
"I was with my family. I was with my wife and our son and had a great time, also trained hard and practiSed hard. I'm probably going to be a little bit more rested than most people, but at the same time, I haven't played competitively in four weeks. So there might be a slight bit of rust, but I'm pretty sure that's going to go quickly just because of how happy and how excited I am to be back and be competing.
"Maybe not this week but looking into the next few weeks, probably physically, mentally, I've been more rested than everybody else. So that could be my advantage."