The absences from the active roster, however, figure to be minimal. With three off-days in the first four weeks, Iwakuma and Walker could miss as few as five starts combined
Iwakuma (sidelined with a sprained ligament in the middle finger of his right hand), Walker (slowed by inflammation in his right shoulder) and Maurer (battling back problems) are all making good on best-case scenarios.
And their absence from the mix this spring has allowed for a longer-than-expected look at revived Blake Beavan and lefty Roenis Elias, the Cuban defector who was a non-roster invite, but is now making a case to jump from Double-A to the big leagues.
"It's always said that one man's loss is another man's gain," Mariners manager Lloyd McClendon said. "When the door is open, you need to take advantage of the opportunity. The young men have done well for themselves."
McClendon joked that Felix Hernandez will make his club-record seventh consecutive Opening Day start, "and I wish I had five more."
Most teams would be happy with one pitcher the caliber of Hernandez, and McClendon is more than happy that he will be the anchor to a rotation in which Erasmo Ramirez is currently the only other healthy member. Rookie left-hander James Paxton has made good on his opportunity this spring, and Beavan "looks like he is back to where he was two years ago," said general manager Jack Zduriencik.
"And [Randy] Wolf, [Scott] Baker and Elias are in the mix for the fifth starter," said Zduriencik of the three non-roster invitees.
Suddenly, the concerns that greeted Seattle at the start of Spring Training, when there were cries from outsiders that Zduriencik had to sign free-agent Ervin Santana -- who has since joined Atlanta -- or make a blockbuster trade for a starting pitcher, have been eased.
Think about it.
Not only have the Mariners found options for the season-opening rotation, but Walker and Iwakuma are on pace to meet the projections that they would both be in the rotation by the end of April.
Walker had a 40-pitch bullpen session on Wednesday, throwing 10 curveballs in the process, "airing it out" at the end and putting himself in a position to face hitters.
"Game situation," Walker said of the final portion. "I felt really good so I let it go a little bit."
And Iwakuma simulated his pitching mechanics while holding a towel, a preliminary to a weekend session in which he will use a tennis ball when he initially begins throwing again.
"We caught a break with him," Zduriencik said. "It was the outer part of the middle finger [and doesn't put as much pressure on the ball]. He just needed to be rested."
The key, said pitching coach Rick Waits, is that Iwakuma continued to do all of his conditioning work, except for throwing, which will expedite his return to the rotation.
That is reason enough for the Mariners to breathe a sigh of relief.
But there is more.
Beavan has shown signs of rebounding from a 2013 in which he made more starts for Triple-A Tacoma (16) than he did in Seattle (two), after going 11-11 for the big league team in 2012.
Beavan allowed only three earned runs in 12 2/3 innings of his first four spring appearances. He became the first Mariners pitcher to work into the sixth inning on Wednesday, giving up four solo home runs in the first five innings, and he was charged with three three more runs while retiring two batters in the sixth.
Wolf, who at 37 is attempting to come back after spending last year recovering from Tommy John surgery, worked four shutout innings against San Diego on Tuesday night, after allowing six runs in nine innings of his three previous spring starts.
Baker has a 3.38 ERA in his first three spring starts, and he is scheduled to go again in Saturday's split-squad game against Oakland in Phoenix.
And then there is Elias, who also will start Saturday, facing Colorado in Scottsdale, Ariz. The 25-year-old Cuban defector is well thought of, which is why he was given a spring invite, but was projected to move from Double-A to Triple-A when camp opened last month.
With the uncertainty in the rotation, however, and after Elias allowed only two runs in 7 1/3 innings of three relief appearances, he gave up one run in five innings against an Angels lineup that featured the projected starting nine, except for Josh Hamilton on Sunday.
That added to the Mariners' interest in seeing him face the offense-minded Rockies lineup.
"The things he's doing don't surprise me," said Waits. "The kid has been on a mission since the first day he came here."
Elias' emergence this spring has made Seattle feel better about its mission in 2014.
"You would like to go through the season with the same five [starting pitchers]," said Waits. "That doesn't happen very often. You need the depth."
And the Mariners appear to be uncovering that depth this spring.
Tracy Ringolsby is a columnist for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.