Hitting the best form of his career at just the right time, Pieters made it tough for Clarke to ignore him. He was selected Tuesday as one of Europe's three wild cards for next month's showdown with the United States at Hazeltine.
The 24-year-old Belgian will be one of six Ryder Cup rookies on a European team seeking an unprecedented fourth straight victory in golf's biggest team event.
"He brings a bit more of the X-factor. He is going to blossom," Clarke said. "He will be a star."
Lee Westwood and Martin Kaymer were "very obvious" choices as two of the captain's picks, Clarke said after making the announcement at the European Tour's headquarters at Wentworth.
He had sleepless nights over his final selection.
It boiled down to Pieters and Russell Knox, players who have won events on different sides of the Atlantic this month.
Pieters got the nod on the back of this hot run: Fourth place at the Olympics, second place at the Czech Masters, and then a victory at the Made in Denmark tournament where he birdied the last three holes to win on Sunday. Clarke was in Denmark to see Pieters' third win in the past year, and made sure he was grouped with him for the opening two rounds.
Then came Clarke's hardest job the phone call to Knox.
"It's one of the most difficult periods in the whole of my golfing career," Clarke said.
Half of the European team will be playing their first Ryder Cup next month in Hazeltine
Masters champions Danny Willett, Chris Wood, Rafa Cabrera-Bello, Andy Sullivan and Matt Fitzpatrick qualified automatically, along with four-time major winner Rory McIlroy, British Open champion Henrik Stenson, Olympic gold medalist Justin Rose and Sergio Garcia.
That partly explains Clarke's desire for more balance to the team, and therefore his call for Westwood a longtime friend and Kaymer.
It will be the 43-year-old Westwood's 10th straight Ryder Cup, while two-time major champion Kaymer has played in the last three Ryder Cups and made the putt that ensured Europe would retain the cup at Medinah in 2012. They will offer experienced voices in the team room.
"Because all our rookies are playing frequently in America now, it's not quite what it used to be," Clarke said. "That being said, the Ryder Cup is not like any other event."
Clarke said Luke Donald another former No. 1 and a four-time Ryder Cup winner and Graeme McDowell would also have been in contention for a pick if their form had been better.
In the end, it was a straight shootout between Knox and Pieters.
Blow for Knox
The U.S-based Knox is a two-time winner on the U.S. PGA Tour this season, including at the Travelers Championship this month, and ranked No. 20 the sixth highest position for a European. His win at a WGC event in Shanghai last year didn't count toward Ryder Cup qualification because he wasn't a member of the European Tour at that stage.
If he had been, Knox would be an automatic qualifier.
"I'm very disappointed not to have made the team," Knox said. "I feel like I deserve to be on the team, but it wasn't my call, obviously. I have to respect the decision he made and get on with it."
Knox said Clarke and Ian Poulter called and urged him to play the Wyndham Championship, his last chance to earn world points. Fitzpatrick played the Czech Masters that week and finished fifth, meaning Knox would have needed to finish fourth at the PGA Tour event to move past him.
Knox said to play the Wyndham Championship would have meant playing eight out of 10 weeks through the Ryder Cup and "I probably would be burned out when I got to Hazeltine."
"You have the right to pick whoever you want," he said. "I gave him that choice by not making the team outright."
Pieters is a big-hitting Belgian who played college golf at Illinois and won the NCAA individual title in 2012 at Riviera. Clarke said he has always been a fan and that Pieters "has the talent to go all the way to the top of the world."
"It's been a little bit crazy," Pieters said. "I was playing well but the results weren't there and I guess I had to step it up at the end. I can't wait to get on that plane and be with the team the whole week."