Rory McIlroy might have been better off skipping the FedEx Cup Playoffs after all following his missed cut in the defence of the Dell Technologies Championship at TPC Boston.

The world No 4 — he could be No 5 on Monday if Jon Rahm converts the 36-hole lead into his third win this year — can only be frustrated with his game and eager for 2017 to come to an end after missing the cut in his defence of a title for the fifth time following the 2011 Wells Fargo Championship, the 2012 US Open, the 2015 BMW PGA and this year's Irish Open.

He also famously walked off the course after completing just 26 holes in his defence of the Honda Classic in 2013.

After adding a 74 to his opening 72 to miss the cut by a shot, McIlroy is projected to fall to 50th in the FedEx Cup rankings with only the top 30 after the third playoff event in Chicago making it to Atlanta. 

As a result, the reigning FedEx Cup champion's US season could be over sooner than expected if he fails to make the Tour Championship following the BMW Championship in Chicago in a fortnight.

He still plans to round off a forgettable, injury-hit season alongside his father in the Alfred Dunhill Links Championship on the European Tour.

But if something doesn't change soon, he looks increasingly likely to finish the year without a win for the first time since his first full season as a professional in 2008.

The 28-year-old made too many basic errors in Boston, taking 33 and 32 putts on the greens, adding another three-putt in round two to the four-putt and the three-putt he had on day one.

Out in one over par on the back nine, his bogey at his 10th hole summed up his day and his season.

A perfect tee shot left him just 94 yards from the pin but he finished up seven yards short of the green, chipped to seven feet and missed the putt.

The four-time major winner was ranked 74th for strokes gained off the tee, 66th from tee to green and 81st for putting.

He was still just one over for the day with five holes to play but made a  double bogey six at the fifth (his 14th) after a poor drive forced him to take a penalty drop.

As for Rahm, he has played just 33 PGA Tour event — five of them as an amateur. But following his breakthrough win in the Farmers Insurance Open in January and his Dubai Duty Free Irish Open on the European Tour in July, he's gunning now to match Dustin Johnson (4), Jordan Spieth (3) and Hideki Matsuyama (2) as a multiple PGA Tour winner this year.

The Basque golfer was two over for the day after eight holes as he followed a bogey at the 12th and a birdie at the 14th with a double bogey five at the 16th, where he took four to get down from a tough lie in greenside rough.

But he erased that mistake with a 200-yard approach to 12 feet setting up an eagle three at the 18th, then roared home in five-under 31.

After taking just 12 putts on his back nine without having to hole anything longer than a 12 and a half footer, Rahm said: " Man, it felt like very different, that front nine and back nine, it was a big difference.

Leading by two strokes on nine under par from Adam Hadwin (65), Paul Casey (65), Kyle Stanley (68) and Kevin Streelman (65), he added: "You know, I don't know what to say. I played a nearly perfect back nine. Felt like I didn't miss a shot. I felt I putted for birdie pretty much from every hole from a really close distance. Nothing to say about that one."

It's been a whirlwind start to his career and Rahm admitted that while he had some lofty goals, he surprised himself.

"Well, to be honest, one of the goals was making it to East Lake, and then hopefully winning a tournament. And you know, making it to the majors and WGCs. And then Torrey Pines, which is one of the first tournaments of the year, I get my win. And then I played the way I did in both WGCs, and I get to 12th in the world going to Augusta.

"Getting to the Top 15 in the world, it's a huge -- it's a long-term goal. It takes people years to get to that spot. So once I got there, my next goal was, okay, let's get into the Top-10. And thanks to that, I kept playing really good golf. I kept making Top-10s, top fives, giving myself chances. Finished top five in Wilmington at the Wells Fargo, and followed by a great week in Colonial. And once I got there, I mean, I got to Top-10 in the world. It really -- I kept adding.

Then I became a European Tour Member; I win on The European Tour. That wasn't even in my mind at the beginning of the year. I'm like, if I can somehow get enough money to become a European Tour Member and get points for The Ryder Cup, it would be amazing. Little did I know I was going to win in the way I did in Ireland and get to seventh in the world.

"Things just kept piling up and I got to the point, like I explained. I felt like for awhile, I had nothing else to play for. I went from two goals to accomplishing many of them."

Q. So what are your goals now?
JON RAHM: Well, I think we all have the same goal in the next few weeks, which is the FedExCup.

Grayson Murray  (68), Phil Mickelson (67), Bryson DeChambeau (67) and Marc Leishman (69) are tied for sixth, three behind Rahm on six under with Jordan Spieth (65) just a shot further back in a tie for 10th.

Justin Thomas (67) and Dustin Johnson (72) are in a seven-way tie for  15th.