Crawley managed scores of nine and 13 in the series opener at Lord’s, in which South Africa triumphed inside three days by an innings and 12 runs.
The 24-year-old, who averages 26 after 26 tests, is now without a half-century in his last 14 innings, which has prompted calls for him to be dropped.
McCullum dismissed suggestions that the Kent batsman could benefit from a break.
“That’s not how I think,” McCullum, whose coaching philosophy, dubbed ‘Bazball’, includes steadfast backing of the players, was quoted as saying by the BBC.
“We want to keep giving guys opportunities, then their skill and talent can come out.
“We have to be really positive around the language we use with him and be really consistent with the selections around that, to keep giving guys opportunities.”
Despite the deflating loss at Lord’s, England’s first under McCullum, the coach said “selection loyalty” was crucial for him.
“Not only does it build loyalty from guys in the side, it also builds loyalty from guys on the outside, because they know when their opportunity does come they will be afforded the same loyalty.”
Crawley smashed a career-best 267 against Pakistan at Southampton in 2020 but has yet to cross 50 since his century against West Indies in March this year.
McCullum put Crawley’s match-winning ability over consistency to justify his faith in the batsman.
“We have some players that have been put in those positions because they have certain skillsets,” said the former New Zealand captain.
“I look at a guy like Zak and his skillset is not to be a consistent cricketer.
“He’s not that type of player. He’s put in that situation because he has a game which, when he gets going, he can win matches for England.”
The second test in Manchester begins on Thursday.