The injury occured today during a sparring session with Billy Evangelista. Thomson will undergo surgery on Friday and is expected to be out two to three months.
Melendez will remain on the card against a replacement to be announced.
"I was sparring hard and I broke my leg, above my ankle, during an exchange," said Thomson. "I'm very upset. All the hard work was done and it was time to fight. I was looking forward to putting on a great show for the fans. All I can say is that I'll be back when I get healthy."
- Frank Shamrock (23-9-2) vs. Nick Diaz (18-7)
- Gilbert Melendez (14-2) vs. TBA
- Scott Smith (14-5) vs. Benji Radach (19-4)
- Cris "Cyborg" Santos (6-1) vs. Hitomi Akano (14-5)
- Brett Rogers (8-0) vs. Ron "Abongo" Humphrey (12-0)
- Marcus Royster (0-1) vs. Yohan Banks (2-1)
- Luke Rockhold (3-1) vs. Buck Meredith (3-2)
- Eric Lawson (8-2) vs. Richard Montoya (6-5)
- Raul Castillo (5-0) vs. Brandon Michaels
- James Terry (5-1) vs. Zak Bucia