There are so many “ifs’’ within the Giants defense the success or failure of the unit so downtrodden a year ago is almost impossible to figure. One of the most glaring “ifs’’ is Jacquian Williams.

Is he an ascending player a young athletic linebacker able to stay on the field for all three downs capable of chasing down a running back and more importantly running with a dangerous tight end? Or is he nothing more than a former sixth-round draft pick who shows flashes but can’t stay healthy enough to make a real difference?

Williams a third-year player whose career has been a promising but frustrating series of starts and stops got some work yesterday with the starting defense subbing for Spencer Paysinger — another third-year player — who had been lining up at outside linebacker with the first-team defense.

Williams also lined up with the No. 1 nickel package along with Paysinger and cornerback Aaron Ross subbing for the third linebacker. This juggling act figures to continue for several weeks although Williams will likely have an edge as long as he can prove he’s durable enough to be there for the long haul.

“I heard a quote Ray Lewis said that ‘What team takes out the general?’ ” Williams said. “My goal is to be the guy that we can count on the guy that you count on you never take him out. I don’t want to come out. I want to be the guy they need the guy they call on the guy they can trust. That’s the goal. With respect to all the other linebackers I’ll do whatever I have to do to be that guy.’’

A torn posterior cruciate ligament in his knee forced Williams to miss six consecutive games last season. He made it back for the last two contests but spent the entire offseason rehabbing and doesn’t look to be moving at his usual breakneck speed. If he’s healthy the Giants want him on the field as he can drop in coverage and deal with tight ends — a valuable ability to possess.

“There’s no question last year was a problem in terms of continuous development’’ coach Tom Coughlin said. “He just missed some time but he’s back on track and I hope he’s going to stay out there. That’s the key for all these people. The thing you must understand is: You’ve got to practice. You’ve got to practice.’’

Given that he is being worked in slowly Williams said he wasn’t surprised Paysinger got the first-team reps at the start of camp.

“Nah it’s not disappointing’’ Williams said. “It’s just this time continue to keep pushing forward my opportunity will come. It’s only what day three day four? It’s time. I have my time to prove myself just like he will too. We all understand coming back from injury is going to take time. I’m happy they just didn’t throw me out there. I had the opportunity to work myself in there.’’