Erasmo Ramirez, the least heralded of the Fab Four prospects, remains with the big club and will start Thursday's Cactus League game against the Giants as the Mariners get a longer look at him.
But Willis knows the four are tied together in many ways, as the quartet has become somewhat of a single entity during the past month while sitting side-by-side in the clubhouse and working in unison on the field.
Willis has had the four pitch on the same schedule, in the same intrasquad games, the same "B" game and then follow each other to the mound again Saturday night at Salt River Fields when they put on an impressive outing with eight innings of one-run, four-hit ball among them against the D-backs.
"It kind of just started when I put Spring Training together," Willis said of their shared experience. "I felt like keeping them together as opposed to having one of them follow Felix [Hernandez] or a more experienced guy.
"I want them to be themselves. I think they've done that and the byproduct of it is they've formed this bond. I walked up last night while Ramirez was playing catch and I got Hultzen, Walker and Paxton standing there all talking together.
"I'm not comparing them, but I have this vision of seeing Smoltz, Glavine and Maddux on the cover of 'Sports Illustrated' playing golf together," Willis said. "They help each other. I just think it's a fluky thing. I didn't plan on them to gain that bond, but I think it's good. I'm hearing they go out to eat together, they cheer for each other and it's huge because they have the potential to be together for a long time, one of these days."
That day will be down the road, because the Mariners have six veterans vying for five rotation spots and won't rush the pups to the Majors quite yet.
Hultzen (No. 2), Walker (No. 3), Paxton (No. 5) and Ramirez (No. 13) are all ranked by MLB.com among the Mariners' top 20 prospects.
Ramirez, a 21-year-old right-hander out of Nicaragua, is the only one of the four who has pitched at Triple-A, and that presumably is why he's still in camp.
Hultzen was the No. 2 overall Draft pick last June out of Virginia, but his only pro experience was a quick stint in the Arizona Fall League.
Walker, 19, was the Mariners' Minor League Pitcher of the Year last year after an outstanding season in Class A Clinton, while Paxton, 23, rose quickly from Clinton to Double-A Jackson last season after not pitching for a year and a half when he didn't sign with the Blue Jays as a first-round holdout.
That future was on display in full force Saturday when Ramirez started and threw three scoreless innings with just two hits, no walks and one strikeout. Hultzen followed and gave up a hit in two shutout frames with one strikeout.
Walker got just one inning, mostly because he's the youngest of the group, and struck out two in a quick 1-2-3 frame. Then Paxton finished things off with two innings of one-hit, one-run ball with two walks and four strikeouts.
"It was hard to not sit there with a big smile on your face," Willis said. "They're young from an experience standpoint. So you know about their stuff, but you're uncertain what you're going to get ... great atmosphere, their first night game, at that facility, Arizona, all that. But those kids pitched liked they'd been there forever. It was a lot of fun to watch, it really was."
Ramirez opened eyes by throwing 23 of his 28 pitches for strikes. At 5-foot-10, he's the least imposing of the four, but that can be deceiving. Willis said he has excellent command of four different pitches and isn't afraid to use them.
"It is impressive," Willis said. "I think the velocity and life on his fastball really surprised people. "
Ramirez said his debut was "fun when it was done" and the nerves had settled. But he said his confidence grew with his "B" game outing against the Reds on Monday when he mowed down six straight hitters and he just carried that into the bigger atmosphere in front of 12,510 fans against the D-backs.
"I just said, 'All right, this is me. I don't care who is hitting,'" he said. "I'll still check out how they're swinging or what they look like at the plate. But just be aggressive. That's what's working for me. Since then, everything comes true. Just be positive and confident in myself and everything can happen."
For his three compatriots, the dream was delayed a bit with Sunday's news. But make no mistake, it's only a matter of time before the youngsters return to the big league club. Which is why Willis was still smiling as he talked about Saturday night's showing 12 hours later.
"We saw good things in the 'B' game against Cincinnati earlier in the week." Willis said. "But to see them go out in that atmosphere last night and step it up a notch, but yet be very much under control, it speaks a lot. Because they're going to have another first time one of these days and it's a bit of an indication of how they're going to handle it."