ARLINGTON— The day belonged to Jacob deGrom and his five-year $137.5 million contract extension.

It was a pretty good Wednesday, though, for rookie first baseman Pete Alonso, who got the official word he made the team and is penciled in to be the starting first baseman Thursday when the Mets open the season against the Nationals and Max Scherzer at Nationals Park.

That’s a challenge for Alonso, who rose to every challenge in spring training, bashing the baseball to the tune of a 1.002 OPS in 22 Grapefruit League games.

Mickey Callaway told The Post that when Alonso was given the news he made the club, “He looked at me with these wide-open eyes as if to say: ‘Everything I have worked for has come true.’ ’’

True to his “players first’’ word, general manager Brodie Van Wagenen did not try to play any roster-manipulation games with the big first baseman.

“He earned it,’’ Van Wagenen said. “Pete is not afraid of competition. He is a hard worker and he has risen to every occasion in front of him. Whether it was the Futures Game last year, it was a big moment, he hits a home run. Whether it was the Arizona Fall League Futures Game he shows up and he hits a home run.

“When he came to spring training he knew that this opportunity was in his control and he seized it. It shows you this guy has prime-time potential.’’

“I’m really happy with how I performed,’’ Alonso said on the team’s last day in Port St. Lucie. “I’m really happy with all the work I got to do to make progress in my game. Looking back from Day 1 I’m really happy with myself.

“I learned a lot about myself. As a player, it’s not necessarily a worry but it’s more just a question: ‘How am I going to do?’ You are not going to know until the dust settles. I feel like I can play at the next level.’’

Van Wagenen said from the start this was going to be a results-only decision.

That caught Alonso’s attention.

“That’s really cool,’’ Alonso said. “That just means that there is an opportunity and I feel like I made the most of my opportunity to earn the spot. That’s what I really wanted to do.’’

Alonso finished with a .352 average, four home runs, five doubles, a triple and 11 RBIs. Alonso could be sent down at a later point but is planning on staying.

DeGrom said he was impressed.

“It’s exciting, this was the first time that I really have been able to see him play,’’ deGrom said. “He’s an exciting player. I think he has got the most power I have ever seen, especially during BP, so that is fun to watch. The kid came in with a goal to win a job and that’s what he did and I am excited to have him at first base.’’