McCullum thinking long-term with out of form opener
Crawley managed scores of just nine and 13 during England's comprehensive loss to the Proteas in the opening Test at Lord's and has yet to register a half-century for his country from 10 innings this summer.
But McCullum has showed trust in the right-hander and believes patience must be shown to the under-fire 24-year-old ahead of the second Test of the series at Old Trafford commencing on Thursday.
"You have to think about the overall package," McCullum said.
"We have some players that have been put in those positions because they have certain skill-sets.
"I look at a guy like Zak and his skill-set 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."
England were outclassed by their opponents during the first Test at Lord’s with South Africa cruising to an innings and 12 runs victory to maintain their position at the top of the ICC World Test Championship standings.
Crawley wasn’t the only England player to struggle against South Africa’s powerful bowling attack, but his run of outs with the bat has seen some commentators call for his immediate axing.
Ex-England captain Mike Atherton was among a group of former players to question Crawley's place in the side, with the retired opener - and now commentator - suggesting a spell out of the team could be to his benefit.
"He didn't look like a man that was going to be there for a long time. England like him and obviously think there is loads of potential but at some point you have to take someone out of the firing line,” Atherton said on Sky Sports.
"The best bowlers put it around off stump time and time again and Crawley is nicking off on and around off stump. That's the game as an opening batsman and if you are struggling to sort that aspect of your game, it is an issue."
But McCullum refuted those suggestions and said the long-term benefits of showing loyalty at selection far outweighs any short-term fix.
"That's not how I think," he said.
"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.
"Selection loyalty is really important. 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."