UFC on ESPN 6 is now missing one-half of its co-main event.

Zabit Magomedsharipov (17-1 MMA, 5-0 UFC) has been forced to pull out of his Oct. 18 bout against Calvin Kattar (20-3 UFC, 4-1 MMA) in Boston.

Two persons with knowledge of the situation informed MMA Junkie of Magomedsharipov’s withdrawal, but asked to remain anonymous since the UFC has yet to make an official announcement. While not set yet, the bout is expected to be rebooked for a future event. Moscow is a leading candidate, though no decision has been made.

Undefeated in the UFC cage, Magomedsharipov would have been looking to knock off his second straight ranked opponent. The versatile Russian striker has quickly ascended up the divisional ladder since his promotional debut in September 2017.

After he submitted Mike Santiago and Sheymon Moraes in his first two UFC contests, Magomedsharipov earned a “Fight of the Night” decision victory over Kyle Bochniak at UFC 223 in April 2018.

Originally scheduled to face Yair Rodriguez at UFC 228 in September 2018, Magomedsharipov instead submitted short-notice replacement Brandon Davis by Suloev stretch.

In his most recent outing, Magomedsharipov defeated perennial contender Jeremy Stephens by unanimous decision at UFC 235 in March.

The withdrawal is Magomedsharipov’s first since he signed with the UFC.

Massachusetts’ Kattar was expected to get the most high-profile fight of his UFC tenure in his own backyard. The 31-year-old featherweight has won four of his five UFC outings with victories over Ricardo Lamas, Chris Fishgold, Shane Burgos and Andre Fili.

Kattar’s most recent fight came against Lamas at UFC 238 in June. “The Boston Finisher” brutally took out Lamas with strikes at the 4:06 mark of the first round. Former UFC title challenger Lamas suffered a broken jaw as a result of the final sequence.

Kattar has won three of his four UFC outings by knockout. His lone loss came by decision to Renato Moicano at UFC 223 in April 2018.

Gallery Zabit Magomedsharipov def. Jeremy Stephens at UFC 235: Best photos view 19 images