The UFC confirmed on Wednesday that the two have verbally agreed to face each other at the Sept. 1 event.
It's a somewhat late booking for a co-feature, considering there is only about six weeks left before the event.
Both fighters will be looking to rebound off recent losses.
Ellenberger had a six-fight win streak snapped in his last match, a knockout loss to Martin Kampmann at the TUF 15 Finale. In that fight, he looked to be on his way to victory after roughing up Kampmann in the fight's early moments, but the Danish fighter turned the tide for the comeback victory.
Prior to that, Ellenberger (27-6) had reeled off an impressive stretch that included wins over Diego Sanchez, Jake Shields and Mike Pyle, among others.
Koscheck lost a split-decision to Johny Hendricks at UFC on FOX 3 in May. Heading into that fight, he had won five of his last six, losing only a UFC welterweight championship match against reigning champ George St. Pierre.
UFC 151 is set for the Mandalay Bay Events Center in Las Vegas. In the main event, Jon Jones will attempt to defend the UFC light-heavyweight championship for the fourth time, facing former two-time PRIDE champion Dan Henderson.
Other announced bouts include Danny Castillo vs. Michael Johnson, Dennis Siver vs. Eddie Yagin, and Takeya Mizugaki vs. Jeff Hougland.