Louis Riddick looks ahead to what's next for the Lions and how Matt Stafford has to play in order to beat the Packers. (0:44)

ARLINGTON, Texas -- The Detroit Lions came into Monday night with a chance to clinch a playoff berth and show they were true NFC contenders. They displayed one thing in their loss to the Dallas Cowboys: They aren't ready for prime time.

The second half of the Lions' 42-21 loss to the NFC top seed in front of a national television audience proved that. This was a game that mattered a whole deal to Detroit. It would have given the team its second playoff berth in three seasons. It would have almost certainly ensured that Lions coach Jim Caldwell would return for another season.

Moreover, it would have taken some of the pressure off Sunday night, when the Lions might essentially be in a playoff game against the Green Bay Packers, depending on whether Washington beats the New York Giants earlier in the day.

That Packers-Lions game is going to be for the NFC North title regardless, but there's a chance it could be win-or-go-home. After seeing how the Lions played against one of the best offenses in the NFL on Monday night at Dallas and one of the best defenses in the NFL in Week 16 against the New York Giants, the feeling in Detroit right now can't be good.

"The whole game, we didn't play well," safety Tavon Wilson said. "They outplayed us today. Just point blank."

It can't help, either, that Detroit led Dallas 21-14 at the two-minute warning in the first half. Everything unraveled for the Lions offensively, defensively and coaching-wise from there. Even Matt Prater, who hadn’t missed a field goal since Week 8 against Houston, misfired on a 47-yard attempt in the third quarter.

What you need to know in the NFL • NFL playoffs coverage, schedule

• Statistics

• Scoreboard

• Complete 2016 schedule, results

• Standings

Quarterback Matthew Stafford, who had played smart football for much of the season, appeared to force some passes and make bad reads. It didn't help that he was pummeled like he had the Lions' 2014 and 2015 offensive lines blocking for him instead of the one that had mostly held together well this season. The Lions' coaching staff also left Stafford in down 21 points late in the fourth quarter, with him being hit nine times by Dallas' suddenly vicious defense.

The run defense, which knew it had a litmus test facing Ezekiel Elliott, couldn't handle the likely NFL Rookie of the Year. Elliott gained only 80 yards, but averaged 6.7 yards per carry, had two touchdowns and, as expected, forced multiple Lions defenders to tackle him. The pass defense, without top cornerback Darius Slay, couldn't make a play on any Cowboys receiver and let Dak Prescott complete 15 of 20 passes for 212 yards, three touchdowns and no interceptions.

"They came out and, you know, had a lot of energy and got the momentum," linebacker DeAndre Levy said. "They came out ready to play and we didn’t. We never really responded. They came out, I think got an interception. We gave up a long drive.

"Everything we could do wrong in that second half, we did wrong. We didn’t capture the momentum back."

It wasn't all on the players, though. The coaching was questionable, too. The Stafford question was already covered, but Detroit went away from its most productive offensive game plan for no real reason. Zach Zenner had 10 carries for 64 yards in the first half, averaging 6.4 yards per carry for a run game that had been invisible for weeks.

He touched the ball only twice in the second half when it mattered, despite looking like the best back Detroit has had since Ameer Abdullah got hurt in Week 2. After the game, Caldwell said "things change" regarding why Zenner had only two second-half carries, noting that the Lions were adjusting to what they saw, which was "a little bit more pass than run." Yet the Lions seemed uninterested in trying to beat Dallas the way they had so many other teams, with sustained drives that involved moving the chains incrementally. Zenner had been a big part of that in the first half.

It was a meltdown by the Lions, who looked overmatched against Dallas. Detroit has now lost two consecutive games and might be in a win-or-season-over situation in less than a week against its biggest rival, Green Bay, at home.

And it left the Lions searching for answers in a game that could have sealed a playoff berth.

"I don’t know, man. It was a big opportunity for us and we had too many mental errors, just misfits here and there and just got caught in a couple looks and they just hit us," Levy said. "A couple scheme things they kind of threw out there for us.

"They came out and played well in the second half and we didn’t. It’s as simple as that."