The province's sidelined players include their Irish World Cup contingent, who are recovering from their exertions in France, and a number of front liners who are currently nursing injuries, including David Humphreys and Paul Steinmetz.
McCall said: "We are pretty stretched at the moment in certain positions and there are a lot of people unavailable for selection. We didn't foresee players like David Humphreys and Paul Steinmetz not being available during these crucial weeks when we are short-staffed.
"But we have great faith in some of the younger players who have come through, they have done an exceptional job in the friendlies and the Ospreys game. There was a collective dip against Edinburgh last week but these are young players having their first shot at this and some of those ups and downs are going to occur."
Niall O'Connor kicked Ulster home in that victory over the Ospreys with four pinpoint penalties and his Harlequins club-mate Cave, who has scored six tries in his last two games for 'Quins, will hope to to follow suit by being a match-winner at Rodney Parade.
Commenting on the Ireland Under-20 Grand Slam winner, McCall added: "Darren is a very promising young player and I think he needs constantly challenged at a higher level than he plays week in, week out in the Ulster Senior League. We have high hopes for him.
"He knows the areas of his game that need improvement but what he needs now is an opportunity and he needs to get onto the big stage and he'll get that opportunity this weekend."
Ulster fans can also expect their first glimpse of Del Fava in competitive action on Saturday, with McCall admitting that the South African-born second row is "relatively fresh and raring to go" after playing two World Cup pool games for Italy.
Former Ireland Under-21 international Hallett will also hope to get some game-time in Wales as back-up at number 10 for O'Connor. The experienced Adam Larkin, who has been the replacement out-half for Ulster's opening two league games, is struggling to be fit due to a niggling groin injury.
McCall rates Hallett highly and is looking forward to seeing how he makes the step-up from club to provincial level. "Kieran joined us in the summer from Bedford on a development contract. He has had a couple of games for Malone and has shown up well and it will be interesting to see how he steps up to the higher level."