Jon Jones doesn’t think Alexander Gustafsson’s retirement is legitimate.

Following Gustafsson’s (18-6 MMA, 10-6 UFC) submission loss to Anthony Smith on Saturday in the UFC on ESPN+ 11 headliner, “The Mauler” left his gloves in the center of the octagon and announced his retirement from MMA.

Gustafsson has since confirmed he is done with competition and intends to move on to the next chapter of life. His longtime rival Jones (24-1 MMA, 18-1 UFC) isn’t believing the comments just yet, but said if he the Swede is indeed walking away for good, he hopes good things for him (via Twitter):

Honestly I don’t believe you but if you’re serious, thank you for everything. You held the division to a standard and made us better. Go kick ass with that Family.

Gustafsson said in the lead-up to UFC on ESPN+ 11, which took place at Ericsson Globe in Stockholm, that if he lost to Smith it would mean he doesn’t have it anymore. He competed well in the fight, but ultimately made a mistake in the fourth round that resulted in him being caught in a rear-naked choke.

The loss follows Gustafsson’s title-fight defeat to Jones at UFC 232 in December. It was an anticipated rematch of a UFC 165 title fight from September 2013 where Gustafsson pushed “Bones” to the limit and nearly claimed the belt.

Outside of Daniel Cormier, Gustafsson is Jones’ biggest career rival. And if the retirement sticks, it appears the champ will be sad to see Gustafsson go.

For complete coverage of UFC on ESPN+ 11, check out the UFC Events section of the site.