The decision had already been made, well before the surprising news Jason Pierre-Paul required surgery to quiet down a herniated disc in his lower back. The plan was for Mathias Kiwanuka to move from linebacker back to his familiar and natural defensive end spot, a move that now takes on greater urgency with Pierre-Paul on the mend.

“The spotlight’s on you,’’ Kiwanuka said yesterday, referring to himself. “It’s an opportunity to shine.’’

The Giants do not want to consider life without JPP, and there’s hope he can return in time for the Sept. 8 regular-season opener in Dallas. That’s no sure thing, though, and coach Tom Coughlin said planning on having Pierre-Paul back for the first game is “an aggressive position to take.’’

No one in the organization views Pierre-Paul’s absence as a long-term situation, but until he returns, the Giants have a ready-made replacement capable of at least holding down the right defensive end spot. Kiwanuka, a first-round pick in 2006, was a defensive lineman until 2007, when a logjam at end prompted the move to linebacker. When Osi Umenyiora missed the entire 2008 season following knee surgery, Kiwanuka switched back to the line and had a career-high eight sacks. He also played defensive end in 2010 before a neck injury cut his season short.

“I’m ready to go,’’ Kiwanuka said. “It’s easy for me to slide back into that position because that’s what I’ve been doing.’’

There is no lack of confidence from Kiwanuka, who said “there’s no doubt in my mind’’ he can pick up where Pierre-Paul left off.

“That’s the position I came into this league playing and I’m looking forward to the opportunity,” he said. “Obviously we’d love to have him back and I’m sure he’ll be back ready to go at a certain point. We’re not going to rush him back for any reason.’’

Pierre-Paul remains in Los Angeles following Tuesday’s microdiscectomy, with an expected recovery time of 12 weeks. He did not miss a game in 2012, but his sack production dropped from 16.5 in 2011 to 6.5. At times he needed epidural injections to sooth his back and allow him to stay on the field.