"When I was called up last September, one thing he told me was he didn't want to think about me being young," said Walker. "He wants me to have the mindset of a 32-year-old."
Walker showed signs of that evolution with two shutout innings in the Mariners' 4-3, 10-inning win over the Padres in their exhibition-season opener on Wednesday afternoon. He gave Seattle a glimpse of that pitcher the club is seeking, who can become the fifth in a rotation already set with Felix Hernandez, Hisashi Iwakuma, James Paxton and J.A. Happ.
The Mariners arrived at Spring Training a year ago with fingers crossed that Walker could make the jump to the big leagues, but it wasn't to be. An overzealous winter lifting program resulted in right shoulder stiffness, and Walker's 2014 was put on hold.
When Walker did get to step to the mound, it was at the Triple-A level. He had a three-start cameo with Seattle in July and then got back to the big leagues in September. That time, it was Walker who sent a message to McClendon.
"He didn't disappoint in September," said McClendon. "Adversity should make him better. He knows he can handle it."
Walker was only 1-1 in the final month of the season, but the loss was a complete game in a 1-0 loss in Toronto. In five appearances with the big club, he allowed only five earned runs in 23 innings. Walker struck out 20 and, more importantly, walked only five.
Walker reinforced the expectations that have developed since the Mariners selected him as a supplemental pick after the first round in the 2010 First-Year Player Draft, the 43rd player taken overall. Scouts raved about his potential.
Seattle wanted to see Walker's maturation.
"You know what potential means?" said McClendon. "It means you haven't done anything yet."
This time around, however, Walker feels like he is ready to cash in and become the starting pitcher the Mariners need to be a legitimate factor in the American League West.
Hernandez, who turns just 29 on April 8, is one of the game's elite pitchers. The five-time All-Star is 88-56 with a 2.73 ERA over the last six seasons and won the AL Cy Young Award in 2010.
Iwakuma, three years removed from pitching in his native Japan, gives Seattle a solid No. 2. In three big league seasons, he is 38-20 with a 3.07 ERA and was an All-Star in 2013.
Paxton has been slowed by a sore forearm this spring, but he is expected to be ready at the start of the season. He and Happ give the Mariners two solid left-handers.
Then there is Walker. He has the ability to be in that Hernandez/Iwakuma stratosphere. Walker had the mid-90s fastball and a hard slider. Now he has added a changeup that he had enough confidence in to use in a high-leverage situation on Wednesday. With one out in the first inning and a runner on third, Walker worked a 2-2 count on Matt Kemp, throwing four fastballs, then he got Kemp to chase a changeup.
"I am throwing it to right-handed and left-handed hitters and know I can throw it any time, in any count, for a strike," Walker said. "I want to have that confidence and feel relaxed out there. My whole confidence right now is pretty good."
Walker never backed down in the exhibition opener. Cameron Maybin led off the game with a single, stole second and took third on a Yonder Alonso groundout. Walker, however, remained in control and after striking out Kemp, he got another two-time All-Star, Justin Upton, to fly out to right.
Walker retired the Padres in order in his second and final inning.
Walker was ranked the sixth-best prospect in baseball by MLB.com prior to 2014, and he was No. 5 on the list the season before.
All that was nice, but Walker is tired of being a prospect. He is ready to do something.
Walker is ready to talk about what he has done, not what he could do. He wants to be a guy Seattle can count on.
There was a hope all that could happen a year ago, but it didn't.
"I was trying to do too much," Walker said. "I've learned to be patient. I feel confidence in myself. What I have to do is go out and get the job done."
Walker got the job done on Wednesday, and it took him only 32 pitches, 19 of which were strikes.
"He's a competitor," said McClendon. "He likes the challenge."
He took that first step in claiming that big league rotation spot he covets.
"I try to caution everybody," McClendon said, remembering back to last spring. "We're all in a hurry to get kids to performance, but it takes time. You have to have confidence in yourself. You have to have knowledge of the league. Those are things that take time."
The Mariners -- and Walker -- would like to think that his time has come.