That's the view of Socceroos boss Tony Popovic after selecting the "fearless" pair in his World Cup squad.
Colorado Rapids defender and ex-Brisbane Roar sensation Herrington, just 18, has thrived since joining the Socceroos, including playing 90 minutes in the 1-0 loss to Mexico at the Rose Bowl on Saturday night (Sunday AEST).
"He can be really good. He's just started out and he's doing so well in (Major League Soccer). He's played every minute of every game," Popovic said.
"He keeps backing up. I don't want to jinx him now - or us - but he's very robust to keep backing up every (game) and they're (Colorado) in every cup competition; it's every three or four days.
"And if you look at their line-up, he's the only centre back that plays in every minute of every game."
Herrington could be in the run-on side against Switzerland on Saturday (Sunday AEST) in San Diego, then potentially start on debut in the World Cup opener against Turkey on June 13.
"In the first training session in March, once I saw him train, he was playing," Popovic said.
"I see that same carefree kid that loves football, has no fear, and he's 18.
"It is quite special at 18 to be doing what he's doing and what he just played in last night. I'm sure he's pinching himself.
"This morning I was sure he would be picked. But if I think about the kid, I'm sure he got up this morning and he was hoping and praying that he's in there, but without a doubt he deserves to be here."
Meanwhile, creative holding midfielder Okon-Engstler, son of former Socceroo and current assistant coach Paul Okon, has brushed aside any questions over his credentials with a stand-out season at Sydney FC.
Okon-Engstler, 21, was originally brought into camp as a Benfica U23s player ahead of last June's crucial qualifiers against Japan and Saudi Arabia, and has gone from strength to strength for club and country.
"He hasn't reached, I believe, half of what he could reach," Popovic said.
"But his improvement over 12 months has been significant in particular physically from where he was at the start of the A-League to now powering away from players, winning more duels, being able to shield the ball better.
"He's always been a technically very good player, taking some attributes after his father there, that's for sure. That was always there."
Popovic believed Okon-Engstler enjoyed a breakthrough with strong performances against Cameroon and Curacao in March.
"I think that gave him a lot of confidence that he belongs at this level," Popovic said.
"You can tell a player they belong, but until they actually reach that ... that's what happens with young players. It can take time."
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