On Day 1 of the Giants’ offseason, general manager Jerry Reese could not have been more forceful and thought-provoking on three salient issues. He addressed the team’s most pressing areas with language that couldn’t have been clearer.

First, he told temperamental Odell Beckham Jr. to knock off the nonsense and grow up. Beckham had one of his worst games Sunday after taking an impromptu trip to Florida on his day off. He expressed his anger over the 38-13 wild-card loss to the Packers and his performance by reportedly punching a hole in a wall outside the Giants’ locker room.

Next, Reese said Ereck Flowers must start playing at the level expected of a first-round pick. Reese even left open the possibility of a position change for the slow-footed left tackle.

Last, and in perhaps the biggest news of the day, Reese left open the possibility that the Giants soon will begin to look at who will succeed Eli Manning.

Let that all sink in . . .

Reese spoke privately with Beckham on Monday, a week after Beckham, Victor Cruz, Sterling Shepard and Roger Lewis Jr. were in Florida. The four, who were pictured shirtless on a boat there, pranced around Lambeau Field without shirts in frigid temperatures before Sunday’s game. They appeared to be answering criticism with an in-your-face statement that they don’t care what anyone thinks, although they insisted they were just getting used to the cold.

After Beckham had only four catches for 28 yards and dropped four passes, he allegedly punched a hole in the wall. The Giants were furious, and Reese said he will be held to account if he did it.

“He’s a smart guy, but sometimes he doesn’t do smart things,” Reese said. “We all had to grow up at different times in our lives. I think it’s time for him to do it now. He’s been here three years now.”

The Giants often have treated Beckham with kid gloves and have refrained from publicly criticizing him, but Reese has had enough.

“I see a guy who needs to think about some of the things he does,” Reese said. “He needs to look in the mirror and be honest with himself. We’ll help him with that, but he has to help himself.”

The Giants need to know whether he can be counted on to be a reliable face of the franchise or if he’s incapable of maturing into a trustworthy teammate. His talent is unmistakable, but if he can’t be a consistently positive influence, the Giants ultimately will have to consider parting ways with him.

As for Flowers, his shoddy pass protection hurt Manning this season. “It’s time for him to show the fruits of being a first-round pick,” Reese said, adding that the Giants will “evaluate whether that is at left tackle.”

Translation: It’s time to move Flowers to right tackle or guard and upgrade the position.

Reese took it a step further, saying for the first time that the Giants need to start thinking about life without Eli. “We always think about every position, but Eli is 36,” he said. “We have started to think about who is the next quarterback, who is in line. So we’ll look into that as we move into the offseason.”

If Reese hadn’t thrown down the hammer on Beckham, this clearly would have been the biggest takeaway. The mere hint that the Giants would consider a replacement for the first time since Manning became the starter in 2004 is major news.

Ben McAdoo said Manning still has a lot of football left in him. There have been definitive signs of slippage, though, and the Giants’ failure to score more than 19 points in any of their last six games is cause for concern.

Reese appeared to be on the hot seat a year ago, but he responded with big free-agent signings and saw a promising first year from McAdoo.

There is more work to be done, but the GM is speaking the plain truth about three issues impacting the team’s short-term and long-term future. There will be painful truths to be faced on all accounts.