Field, whose father Jamie is a former Leeds forward, joined the club from Wetherby Bulldogs, while Sinfield, son of former Rhinos captain Kevin, is a Saddleworth Rangers product and Simpson played his junior rugby for Kippax.
A stand-off/back-rower, Field captained Rhinos’ academy this year and Agar described him as “a natural footballer, a really skilful rugby league player”.
He said: “He is a kid who would have played a fair bit at six as a junior, but he has got the size and frame to develop into a back-rower too.
“That could be as an edge player, he has a skillset that could easily be utilised on an edge, or as a ball-playing middle.
“We like that versatility in Oli. He offers something as a back-rower, rather than being a guy who’s all about power and industry.
“He has got some real subtleties and natural footballing ability.”
Agar added: “He is a super kid and will really benefit from full-time training.
“The academy have been heavily disrupted over the last couple of years, but he has been a good performer for them.”
Sinfield has been awarded the number 32 jersey his record-breaking father wore for his debut in 1997.
Agar admitted the famous name will put extra pressure on the rookie scrum-half but stressed: “Internally it’s not something we talk about.
“We understand the natural comparisons; that’s going to happen, but when he’s with us he’s not Kevin’s son, he’s Jack and we take him on his merits.
“He is a super young player and a terrific young man and we all know Kevin well enough to know he would not want Jack judging any other way.”
Agar confirmed: “Jack had plenty of opportunities both in league and union, but he really wants to play for Leeds in rugby league.
“He has got a terrific skill set and a terrific temperament.
“He is a genuine, on the ball number seven; he kicks the ball really well, he’s a natural striker of the ball and kicks it a long way.”
The crucial role he plays means Sinfield will not be rushed into first team action, Agar stressed.
“For half-backs coming into [the senior squad] the game is very different,” he warned.
“It is probably a position that changes more than any other from when you are playing as a junior.
More - Super League Magic Weekend fixtures confirmed: Here's who Leeds Rhinos, Castleford Tigers and Wakefield Trinity will play“It is very different when you are playing against organised, systematic defences; you still want all those traits and instinctive play, but you have to understand cause and effect and read numbers and triggers in defensive lines.
“It is very different from what he would have experienced at junior level.”
Rhinos began pre-season yesterday and their first on-field training session was scheduled for this morning, but Sinfield has already been in to work with newly-signed half-backs Aidan Sezer and Blake Austin.
Agar said: “That will be good for Jack, working with some experienced players. He is a highly intelligent kid, with a terrific temperament.”
Simpson is the latest addition to Rhinos’ full-time squad, having signed a four-year deal last week following fellow centre Corey Hall’s transfer to Wakefield Trinity.
Agar predicted: “He is going to be a really good player.
“He has a brilliant attitude, he comes from a really good family.
“He is really well skilled, he is tough and he carries the ball well.
“We’re very excited about him.”
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