That's not true actually, as Zack Grienke throws 89 mph fastballs and 87 mph changeups. He has lost velocity on his fastball over the years but his changeup velocity has stayed the same. That's actually pretty common. The same thing happened to John Danks, whose changeup was his best pitch but an 84 mph changeup when you're throwing 88 is different than an 84 mph changeup when you're throwing 94.
Generally you want an 8-11 mph separation between the fastball and change. Right now Crochet has it at 4-5.
He has said that he's working on his change grip because(according to Crochet himself his change is 87-88 mph during bullpens but during games it ends up 92-93 mph)
that would be a more appropriate velocity gap.
However, pitchers that throw upper 90s can get away with 90-91 mph changeups.