The Americans will start their bid for a first victory on European soil since 1993 with Spieth and Thomas, who have won four points from six matches as a pairing across the last two Ryder Cups, watching from the sidelines.
The duo also won all four of their matches together at the Presidents Cup last year.
But Thomas was a controversial captain's pick by Johnson after a poor season in which he missed the cut at three of the majors and finished tied-65th at the PGA Championship, where he is a two-time champion.
He also failed to qualify for the PGA Tour's season-ending FedEx Cup playoffs.
Three-time major champion Spieth has not won a title in 2023 and has missed six cuts.
"We've got 12 guys. Unfortunately, I can't play all 12 each session," US skipper Johnson told reporters at Marco Simone Golf and Country Club on Thursday.
"So at some point, somebody's got to sit. It's a golf course that demands a lot out of you physically.
"I think it's an ideal situation where you don't necessarily want to play everybody all five sessions...
"I have the utmost confidence in these eight (who were selected) and the utmost confidence in Jordan and Justin.
"I know we're talking about a great tandem, but it's a situation where it's not about their form. They're playing great."
Johnson has opted to send out three of his four rookies for the foursomes, with Sam Burns playing in the opening match alongside Scottie Scheffler against Jon Rahm and Tyrrell Hatton.
Burns' fellow debutants Max Homa and Brian Harman will face Scandinavian duo Viktor Hovland and Ludvig Aberg.
"History will show, and you can look it up, that being a rookie is almost irrelevant," said Johnson. "So zero hesitation."
The four US players sitting out of the foursomes have 11 major titles between them - with five-time major winner Brooks Koepka and reigning US Open champion Wyndham Clark joining Spieth and Thomas in being benched - seven more than the rest of the team combined.
Koepka, the only player from the Saudi-backed LIV Golf tour who will play this weekend, was met with a handful of jeers among the applause when his name was read out at Thursday's opening ceremony.
But Johnson is confident that any potential heckling from the home crowd will not affect Koepka.
"I know Brooks well enough to know that he's very secure in himself, and I know that his 11 teammates are very secure in him. So the way we approach it; that's all that's important," he said.
When asked if the four players left out of the foursomes would play in Friday afternoon's fourballs, Johnson only said they would be "playing Marco Simone this week".
