The Mariners took another step on Thursday that indicates Julio Rodríguez is all the closer to his Major League debut. The club added its top prospect to the 40-man roster, along with outfielder Alberto Rodriguez (No. 21 prospect) and left-hander Ray Kerr to bring that total to 39.

Why Thursday? It’s within one day of the deadline for clubs to add eligible players to their 40-man to protect them from being selected in next month’s Rule 5 Draft.

• Highlights from Rodríguez's career

The Rule 5 allows clubs without a full 40-man roster to select certain non-40-man roster players from other clubs, and clubs pick in reverse order of the standings from the previous season. Players signed at age 18 or younger need to be added to their club's 40-man within five seasons, and players who signed at age 19 or older need to be protected within four seasons.

The Mariners have been fairly active participants in the Rule 5 in recent years, especially in the reliever market, where they have uncovered under-the-radar talent such as Will Vest, Yohan Ramirez and Brandon Brennan.

But back to Thursday’s transactions.

Julio Rodríguez, MLB Pipeline’s No. 2 overall prospect, will enter Spring Training with arguably as much intrigue as any Mariners player, given that he will soon turn 21 years old and has been touted as having tremendous potential.

J. Rodríguez said at the end of the regular season that he has every intention of competing for an Opening Day roster spot, and Mariners president of baseball operations Jerry Dipoto didn’t dispel the reality of those ambitions, though he did indicate it would hinge on what players the club adds from outside the organization this offseason. Dipoto even hinted that Rodríguez, who has come up as a right fielder, could be a long-term answer to Seattle’s big question in center given his improved speed and athleticism.

• Meet J-Rod, MLB's next superstar

Rodríguez hit .347/.441/.560 (1.001 OPS) with 13 homers last season between High-A Everett and Double-A Arkansas, along with a month-long stint in the Tokyo Olympics, where he won a bronze medal with the Dominican Republic.

A. Rodriguez, the key return in the 2020 Trade Deadline deal with Toronto involving Taijuan Walker, jumped from Low-A Modesto to High-A midway through last season, hitting .289/.379/.470 (.849 OPS) with 10 homers and 65 RBIs. The Mariners really like his consistent contact, quickness to the ball and strong recognition of the strike zone, and they see him as a part of their future.

Kerr jumped from Double-A to Triple-A Tacoma and possesses 100 mph velocity, making him intriguing enough to keep for now. He compiled a 3.18 ERA over 39 2/3 innings with 60 strikeouts among 163 batters faced for a whopping 36.8% strikeout rate. He’ll likely be on the Mariners’ Top 30 prospects list in the next rankings.

By adding the three prospects, the Mariners now have only one open 40-man spot, which almost certainly will change as they navigate what they’ve said will be an active offseason that will include spending, which means they’ll have to make other decisions on players they keep or part with ahead of Spring Training. The next big date on the calendar to do so is the non-tender deadline on Dec. 2.