Dave Meltzer breaks it down:

...at 31, Faber is in the most danger of losing his status as the WEC’s marquee star since the company’s relationship with Versus started. He’s 22-3 but has lost two of his past three fights, both to former champion Mike Brown.

A win would lead to what could be promoted as the biggest event in company history – perhaps even the WEC’s first pay-per-view main event, as has been discussed internally. A loss would for the first time remove him from title contention. Assuncao is a submission expert, having submitted eight opponents, and is particularly dangerous with the armbar.

...

Faber’s big fights of the past two years – a 2008 match with Jens Pulver and June’s featherweight title rematch with Brown – were the two biggest money fights in WEC history. He retained his title by winning a decision over Pulver in the former. In the latter, Faber lost a decision in a match high on most lists as match of the year in a match where Faber injured both his hands, which had been sore for a long time coming off the Pulver fight.

The Pulver match drew 1.6 million viewers and the Brown fight on June 7 drew 1.3 million. Those numbers on Versus are even more impressive when you consider that no fight without Faber has done more than 700,000 viewers, and the most recent WEC show on Dec. 19 (featuring Donald Cerrone vs. Ed Ratcliffe) did 333,000.

Faber's seeming inability to beat Mike Brown couldn't have come at a worse time for Faber and the WEC. After more than two years of reigning at the top of the featherweight division, Faber dropped two straight fights to Brown just as his years of hard work had begun to pay off and attract a large audience.

And as the numbers Meltzer cites show, the WEC has no one else remotely comparable to Faber as a draw. For the sake of the WEC, Faber really needs to win this fight.