"I found out just watching TV one night," Manning said Monday on the first day of offseason workouts, according to Giants.com's Michael Eisen.
"It just came across the (ticker) at the bottom of the screen. Started to get a few texts. That is how I found out."
Manning and Beckham spent five seasons together. The star wideout eclipsed 1,000 yards in every year in which he played at least 12 games with the Giants.
Manning is entering his 16th season with the Giants, a franchise he's won two Super Bowls with. Still, he wasn't extended a cursory call from the team about the impending trade of its superstar talent.
The quarterback said his immediate thoughts turned to his former teammate.
"The reaction, I wanted to reach out to him. Odell has been a teammate for five years and has been a friend," Manning said. "These circumstances, you think about the player involved. It is more than just a player; it is someone you have a relationship with. You think more about that person, how they are handling it, and that they are OK and doing well with it."
Beckham is said to have struggled with the news upon learning he was traded and even announced weeks after that he would take a break from social media to "gather" his thoughts before eventually deleting the post.