SEATTLE -- Jerry Dipoto is well aware of the fan perception that the Mariners need to add to their starting rotation in the current free-agency period, but Seattle's general manager noted on his latest Wheelhouse Podcast on Wednesday that last year's late-season additions already have greatly boosted the pitching staff.

Dipoto chose to spend this offseason beefing up the bullpen rather than the rotation, believing the starting staff is already in better hands than people think with Mike Leake and Erasmo Ramirez , as well as versatile right-hander David Phelps .

The Mariners acquired Ramirez and Phelps just prior to the July 31 non-waiver Trade Deadline, while Leake came in a deal from St. Louis in August.

• Listen to the Wheelhouse with Jerry Dipoto

An elbow injury sidelined Phelps shortly after his arrival, but Leake and Ramirez both pitched well in the rotation and now help fill out a group that includes returners James Paxton , Felix Hernandez and Ariel Miranda , plus youngsters Marco Gonzales and Andrew Moore .

Dipoto acknowledged again that the "heavy lifting is done" for the offseason and the club will primarily look to add a few veteran non-roster invitees before the start of Spring Training next month, though he also allowed that his philosophy will always leave room for further moves as needed.

"We're looking at it as we had Phelps for 10 innings, Leake for five starts and Erasmo for roughly a third of the season," Dipoto said. "You add the three of those guys to our pitching staff and expect a full, healthy season out of each of them, and I feel we've taken a fair step forward with our pitching without doing much else.

"And we're always open to finding ways to get better. Not just today on the first week of January, but we're trying to find ways to get better all season long, as can be seen by the timing of those acquisitions."

Dipoto noted that the trend around a slow-moving free-agent market around MLB has been to spend more on relievers this winter, and he credited the Rockies with what he called a "brilliant" strategy of using their available money where it had the most impact by creating a powerhouse bullpen to go with a young rotation.

The Mariners have more veteran presence than the Rockies in their rotation with Hernandez and Leake, but they are following a similar strategy in investing more on relievers.

"I feel pretty good about our team, our bullpen in particular," Dipoto said. "Having Phelps, [Nick] Vincent and [Juan] Nicasio makes us the only American League team with three setup men who are all coming off seasons of 20 or more holds. The only other team in the big leagues with the same claim is the Rockies.

"So our setup crew, we feel pretty good about. Our young arms -- James Pazos , Dan Altavilla , Nick Rumbelow among others -- really excite us. We have a lefty in [Marc] Rzepczynski and the ninth inning covered with what we feel is one of the better young closers in Edwin Diaz ."

Among the numerous other topics covered in the Podcast:

• Top prospect Kyle Lewis will not be among the invitees to Major League camp this spring as the club looks to continue bringing him along slowly as he returns from knee surgery that limited him to half a season of play in 2017.

• Dipoto said having 12 night games in Cactus League play this spring is by design as the club looks to work in more "rest and recovery" time for players and allow more specific morning workout time for groups and individuals.

• The GM marveled at the natural ability of hitting coach Edgar Martinez, who just turned 55, to still pick up a bat and deliver on cue.

"He can walk into a batting cage with no prep, no stretching. He'll just grab a bat and hit a line drive somewhere. And he'll do it over and over," Dipoto said. "It's like riding a bike, watching what he does. I can go in there and hit a couple balls off the top of the screen and foul 'em off, and he'll go hit rocket after rocket into a gap."

•The Mariners' coaching staff is meeting this week at Safeco Field to begin establishing the plan for Spring Training, and Dipoto talked at length about baserunning, how metrics are able -- or not always able -- to accurately measure good baserunning and the focus his club will place on improving in that area.

Greg Johns has covered the Mariners since 1997, and for MLB.com since 2011. Follow him on Twitter @GregJohnsMLB.