NFL

A BIG DIEHL

This is a test of the emergency blocking system.

Make that the final real test for Giants lineman David Diehl, the last time he’ll take the field for an extended period before the start of the regular season, when he’ll be entrusted to preserve and protect the health of Eli Manning.

“That’s where this third preseason game comes in play,” Diehl said. “I think this is definitely the game I’m going to feel the best. I have the confidence in myself and I know I have the ability.”

The Giants had enough confidence Diehl could make a successful transition from left guard to left tackle that following last season they parted ways with longtime starter Luke Petitgout. Tonight, Diehl and the rest of the Giants starters will play into the third quarter during their annual preseason tussle with the Jets. After that, it’s a cameo appearance in the preseason finale in New England.

Tonight is the last tune-up before the hits start coming for real Sept. 9 in Dallas.

Diehl this summer was given a crash course in what it takes to hang in at left tackle, when day after day he lined up opposite Osi Umenyiora. Sometimes he battled the cat-quick defensive end to a standoff. Other times it was no contest, with Diehl backpedaling not nearly fast enough to prevent Umenyiora from charging unimpeded into the collapsing pocket.

“David’s a very good tackle, he plays well, but you just can’t expect him to block me one-on-one for a whole practice,” Umenyiora said. “That’s kind of crazy.”

With the Giants and Jets scheduled to face each other in the regular season (Oct. 7), figure there will be even less shown tonight than usual, but Diehl will get a look at a few defensive wrinkles.

“They will have game-planned, they will give us the speed rush on the outside, they will have advanced just as we have their nickel, dime package, that’s all part of this,” Tom Coughlin said. “This is a great time to let all that stuff happen; hopefully he’ll do well with it. We don’t want the quarterback hit, obviously. He’ll do well with it.”

Jobs are there to be gained and roles need defining. With cornerback Sam Madison out with a pulled hamstring, Corey Webster starts, which he might have to do against the Cowboys if Madison isn’t ready. Webster started 10 games last season but wasn’t particularly impressive battling through a painful hip injury, which has healed following surgery.

“We prepare like starters anyway,” Webster said. “I’ve been preparing like a starter, so it’s nothing new.”

With tight end Jeremy Shockey (hamstring) likely to sit out, there will be loads of snaps for rookies Kevin Boss and Michael Matthews.

The place-kicking derby, which at the moment seems to have swayed toward young Josh Huston, should come into focus with veteran Lawrence Tynes expected to get the bulk of the field-goal work.

This will be another game for the Giants to view life without Michael Strahan, who remains out of the picture as he contemplates retirement. Justin Tuck will start at left defensive end.

Punter Jeff Feagles will not suit up because of back spasms, leaving those duties to Cory Ohnesorge, a first-year player from Occidental College. Manning will be followed in the quarterback rotation by Anthony Wright, then, if there’s time, Tim Hasselbeck.

“It is a game where the starters are going to play into the third quarter, so you’d like to go in there and be sharp and play a good game,” Manning said. “It’s always a big game between the two teams, it will be a lively stadium; it is important to go out there and try to be sharp.”

paul.schwartz@nypost.com