The biggest decision the Giants make on Thursday night when they prepare for their first-round selection in the draft won’t have much to do with who is on their board. First, they must look within.

They need to decide if their quarter-of-a-billion-dollar face lift on defense so far this offseason is enough to fix the unit that floundered at the bottom of the league rankings in almost every category last year and was a large culprit in a third straight losing season, or if there are more immediate impact pieces needed to support that side of the ball.

Most people believe they will go defense, where there undoubtedly will be enticing options at pass rusher, linebacker and the secondary.

“Quite honestly, I don’t think there’s any chance the Giants take an offensive player,” ESPN analyst Louis Riddick, a former NFL player and front-office director, said on a conference call last week. “I just don’t see it happening, based on what has allowed them to have success in the past and what they have been deficient of in the recent past, that being pressure on the quarterback and high-level linebacker. I think that’s where their emphasis is going to be. That’s where their emphasis was in free agency, and that’s where their emphasis is going to be in this draft, particularly with that first pick.”

General manager Jerry Reese suggested that the splurges of the past month and a half will have no bearing on which direction the Giants go this week.

“We’re not oblivious to what we did in free agency,” he said last week, “but the draft stands alone.”

The Giants have drafted defense in the first round only twice in the past seven drafts and haven’t taken any defenders there since Prince Amukamara in 2011. The expectation is that they will go against that trend this week, but there could be scenarios that alter that.

So what would it take for the Giants to select an offensive player?

A gift likely would have to freefall to them — picture running back Ezekiel Elliott sitting there at 10 — or the Giants either could trade back in the first round or move up with their second. Moving up could be costly, considering their need for impact players, but if Reese sees this draft as his last chance to keep his job, he might be willing to sacrifice future picks in exchange for immediate results.

The Giants do have offensive needs. They have no clear No. 2 wide receiver and the right side of the offensive line could use an upgrade. They have selected an offensive lineman in the first round in two of the past three years and a lineman in either the first or second rounds in each of the last three years. That’s helped stabilize the left side, and the project could continue.

Reese said he would not rule out drafting at any position.

“We’re looking at it that we need help on offense, we need help on defense, we need help on special teams,” he said, “and we’re trying to get good players in every aspect of those positions.”

The need, though, is on the defensive side. Barring an unexpected change in the way the draft develops, it’s a need the Giants figure to help plug on Thursday.