To clear the roster spots for Ramos and Hjelle, the Giants returned Rule 5 selection right-hander Dedniel Núñez to the Mets and designated right-hander Jay Jackson for assignment.
The Giants' 40-man roster is now full.
Ramos, known for his power-speed combination, has shown the ability to handle center field, though he might profile best on the corners. Either way he's in line to be part of the team's outfield mix next season after finishing 2021 at Triple-A Sacramento.
Ramos, 22, ranked as the team's No. 4 prospect per MLB Pipeline.
Hjelle opened the 2021 season with Double-A Richmond, where he had a 3.15 ERA over 14 starts. He struggled a bit after being promoted to Sacramento with a 5.74 ERA in 10 starts, but president of baseball operations Farhan Zaidi has already said he expects the 6-foot-11 right-hander to make starts for the Giants at some point in 2022.
The 24-year-old Hjelle's presence is particularly important given the Giants are short on starting pitching depth with Kevin Gausman, Alex Wood, Anthony DeSclafani and Johnny Cueto hitting the free-agent market. Hjelle was ranked as the No. 13 prospect in the organization per MLB Pipeline.
Players first signed at age 18 or younger must be added to 40-man rosters within five seasons or they become eligible to be taken by other organizations through the Rule 5 Draft process.
Players signed at 19 years or older have to be protected within four seasons.
Clubs pay $100,000 to select a player in the Major League phase of the Rule 5 Draft. If that player doesn't stay on the MLB roster for the full season, he must be offered back to his former team for $50,000.
For this year, that means an international or high school Draft pick signed in 2017 -- assuming he was 18 or younger as of June 5 of that year -- had to be protected. A college player taken in the 2018 Draft was in the same position.