The Dolphins defense this year has taken all assumptions and tossed them in the incinerator.

They are one of the best pass-rushing teams in football, yet No. 3 overall pick Dion Jordan has just two sacks.

They have three of the league’s top 16 defensive tackles, according to Pro Football Focus, yet rank 25th against the run, allowing 121.8 yards per game.

Their pass defense, a huge concern entering the season, has been without starting corner Dimitri Patterson for all but five games. And yet, they’ve allowed the third-fewest passing touchdowns in the NFL (12).

But no one captures the up-is-down nature of the 2013 Dolphins like Dannell Ellerbe, the high-priced free agent linebacker brought in shore up the middle of the defense.

Ellerbe in 2012 was the league’s third-best inside linebacker against the run, Pro Football Focus found. But in 2013, he’s one of the worst.

But on the flip side, he’s gone from a liability in coverage to a very good pass defender.

No better example of this was in the Dolphins’ 23-3 thumping of the Jets on Sunday. Ellerbe was so close to receiver Greg Salas on a first-half passing play, he was essentially a second jersey.

But Geno Smith threw it anyway, and Ellerbe caught the pass as if it was intended for him.

“I guess it was a birthday present,” said Ellerbe, who turned 28 last week. “I ain’t think he was going to throw it. I was surprised that he did.”

It was the kind of big play the Dolphins envisioned when they gave Ellerbe a five-year, $35 million contract back in March. They just envisioned he would make more of them.

Ellerbe is second on the team in tackles with 77 (according to team-provided statistics), but only two have been for loss. He has missed 12 tackles this year, seventh-most among inside linebackers and 21st-most among all NFL defensive players.