A big-time NFL Christmas doubleheader kicks off Monday afternoon at 4:30 p.m. ET with a matchup between the Pittsburgh Steelers and Houston Texans, a game that could have big implications at the top of the AFC standings. The Steelers are nine-point favorites after opening up at -10.

The Over-Under, or total number of points Vegas thinks will be scored, is 45, up one point from where it opened.

With playoff seeding on the line and the odds on the move, you need to hear what SportsLine expert Mike Tierney has to say.

A veteran sportswriter with decades of experience covering the NFL, Tierney studies matchups like nobody else, and he's used his expertise to crush NFL picks. After yet another cash on the Under in last Saturday's Chiefs-Chargers game, he's now an insane 48-22 on NFL totals since the start of last season. Anybody following his picks is way, way up.

Now, Tierney has analyzed every matchup, every trend, and every angle for Pittsburgh-Houston and locked in his Over-Under pick.

Tierney knows that the Steelers have some serious offensive threats, led by running back Le'Veon Bell, who has 307 carries for 1,222 yards and eight touchdowns this season. And even with receiver Antonio Brown (calf) sidelined, Pittsburgh still has plenty of options for Ben Roethlisberger, including JuJu Smith-Schuster (699 yards) and Martavis Bryant (478 yards).

They've scored at least 23 points in their last five contests and have had some offensive explosions during that span, including 39 points against a tough Baltimore defense in Week 14 and 40 points against the Titans in primetime in Week 11. 

SportsLine's advanced projection model says Roethlisberger will roll on Monday, throwing for almost 300 yards and two touchdowns. Smith-Schuster is the best receiving bet to find the end zone.

Roethlisberger has thrown multiple TDs in six straight games, and the Steelers have won eight of the last nine games outright. And in their last four wins, they've scored an average of 33 points.

The Texans, decimated by injuries and already looking ahead to next year, come into Monday's game dead last in the NFL in scoring defense. Pittsburgh has the potential to score in bunches in this one.

But that doesn't necessarily mean that this game will go Over 45.

Though Pittsburgh does have other options, the loss of Brown can't be completely overlooked. He leads the NFL in receiving and is the second-most targeted player in the league. Without him on the field, Houston should be able to devote more players to stopping Bell and the running attack. 

Largely without Brown last week, the Steelers only scored 24 against the Patriots.

And the Texans likely won't be able to contribute too many points in this matchup. They've reached 20 just once since quarterback Deshaun Watson went down and now they're down to third-string signal caller T.J. Yates. Overall, they've averaged just 12 points in their last three contests. 

SportsLine's model says Yates is twice as likely to toss an interception on Christmas Day than a touchdown, so it could be tough sledding for Houston's offense.

Tierney has evaluated all of these angles and he knows there's an alarming stat that ultimately determines whether Texans-Steelers goes Over or Under. And he's sharing it over at SportsLine.

So which side of the total should you back in Steelers-Texans on Christmas Day? Visit SportsLine now to see what alarming stat gives a huge clue as to whether Steelers-Texans goes Over or Under, all from the expert cashing on almost 70 percent of his NFL totals picks since the start of last season.