He couldn't KO Weidman, who unlike Jacare can't fight going backwards for shit, doesn't keep a tight and high guard, and isn't much of a kicker either. Weidman's durability is also almost completely shot.
Jacare will ground the smaller Gastelum, slice through his guard like butter and choke him in the first round. This is a pretty bad mismatch. Jacare is like Weidman but better in every area minus takedowns, and Gastelum has pretty bad TDD (taken down by nearly everyone he's fought, even stick-thin WW Magny) so he won't put up much resistance.