Warren Gatland admitted on Monday that there was an element of risk involved after bucking convention and naming only two scrum‑halves in Wales’s World Cup squad.

In the only contentious call among the squad of 33, Gareth Davies and Tomos Williams will not be joined by Kieran Hardy, who started on Saturday in the 52-16 defeat by South Africa in Cardiff.

It means the 22-year-old fly-half Sam Costelow will serve as an emergency scrum-half if needed. He attended Oakham School alongside England’s injured scrum-half Jack van Poortvliet, and at 5ft 8in he has the build of a regular No 9.

While acknowledging the calculated gamble, the Wales head coach said the decision had been made in light of a few players in the back three who were “getting on a bit”.

Gatland said: “We’ve wanted a bit more cover in the back three. If we do pick up injuries, it’s not too far of a distance to go [to bring in a replacement].”

The side will be co-led by the inexperienced duo Dewi Lake and Jac Morgan. Four years ago they shared a field for the Welsh under‑20s and now, with only 20 Test caps between them, the Ospreys pair have their sights on a World Cup final.

“There’s not much point going if we don’t have the belief that we can achieve that,” Lake told the Welsh language broadcaster Newyddion S4C shortly before Gatland played down that fighting talk. “It’s one step at a time,” Gatland said. “You don’t look too far ahead. We’re just focused on getting out of our pool.”

Wales’s first game is against Fiji and Gatland has been assured that Gareth Anscombe and Taulupe Faletau will be ready despite carrying niggles that have kept them out for the duration of the summer series.

The same was not true for Taine Plumtree, the lock who was forced off the park against England last week with a shoulder knock. “He’s really unlucky,” Gatland said of the second-row.

“Looking at the next couple of weeks, looking at players who can’t take part in live training sessions, he’s unfortunate to be the one who’s drawn the short straw. I chatted to him this morning and said, ‘be ready’, because there’s always potentially an opportunity.”

Plumtree’s absence means Gatland has selected three genuine second‑rows with Christ Tshiunza serving as a hybrid player who can cover multiple positions in the pack. The 21-year-old has shone for Exeter Chiefs but has yet to prove himself at the highest level. “Long term we see him as a six that can cover the second row,” Gatland said.

George North will play in his fourth World Cup and is one of four players with more than 100 Test caps alongside Dan Biggar, Leigh Halfpenny and Faletau. The co-captains Lake and Morgan both said that they would lean on that experience as much as possible. “They’ve been great in camp,” Morgan said of the veterans around him. “There’s plenty of leaders within our squad who share their opinion.”

There were questions as to how the co-captaincy would work on the field. What would happen, for example, if one points to the poles after a penalty call and the other starts marshalling the pack for a lineout maul? Gatland explained that one would take the lead on a particular day and this would change depending on the opposition.

The 59-year-old head coach also revealed that none of the unselected players will remain with the squad, which is a departure from the way other teams have retained non-playing reserves, and encouraged them all to return to their domestic regions and maintain their form and fitness.

Wales have been placed on the easier side of the draw though they are far from guaranteed safe passage from their group. After Fiji they face Portugal before playing Australia and Georgia, two teams that beat them last year.

• This article was corrected on 3 September 2023 to remove a reference to Taine Plumtree being born in New Zealand. Plumtree was born in Wales but had played for New Zealand under-20s, qualifying through his father.