The one player on defense the Giants felt they could least afford to lose was middle linebacker Jon Beason. Now they’re facing the possibility that they’ve lost him for Opening Day, and maybe more.

Beason was diagnosed with a torn ligament and a fractured sesamoid bone in his right foot after he injured it during practice on Thursday morning. He will be examined by Dr. Robert Anderson, a foot and ankle specialist, in Charlotte on Monday. But according to a team source, the Giants expect he’ll need surgery and at least three months to recover.

That means Beason is likely to miss all of training camp this summer and, if all goes well, will be on track to return to practice about the same time the regular season opens on Sept. 8.

“The prognosis is X amount of time and (the opener) is within that timeframe,” Beason said in quotes released through the team. “I expect to be back (for the opener). If not, I’ll be back as soon as I can. That’s really how you have to look at it. If it’s not 16 (games), maybe it’s 15 or 14. Whatever it is, you want it to be that number as opposed to one.”

That’s hardly a guarantee that he’ll be ready for the start of the season, but Beason knows better given his long history of injuries. He missed all but one game in 2011 with a torn Achilles and all but four in 2012 with knee and shoulder injuries. He needed microfracture surgery on his right knee after that season ended, and it was still affecting him early last season when the Carolina Panthers traded him to the Giants for a seventh-round pick.

Beason’s health was never an issue with the Giants, though. In fact, he became one of their best defensive players and earned himself a three-year, $16.8 million contract in March. His return was considered a key element to the Giants’ rebuilding plan, especially considering the leadership void left in the locker room when the Giants failed to resign Justin Tuck.

But that void became even bigger when the 29-year-old Beason stepped wrong while covering a pass play in practice and, he said, “felt like I stepped in like a sprinkler head hole. I just felt it give right away.” He began to limp off the field and was then carted to the locker room and taken to the Hospital for Special Surgery in Manhattan where he underwent and MRI and a CT scan and what he called a “freak injury” was diagnosed.