Broadbent, 20, has played a major role in their campaign so far, missing only two of their dozen games.
Equally adept at full-back, wing or centre, he has certainly taken his chance to shine during the club’s various issues with injuries, suspensions and Covid, scoring a hat-trick in the recent win over Leigh Centurions.
However, Batley-born Broadbent limped off in the first half of Monday’s impressive 22-16 success at Warrington Wolves which extended Rhinos' winning run to four games.
He is due to see a specialist and may yet need surgery.
Agar said: “It is a pretty serious injury; we are looking at initial estimates of 12 weeks.
“He is in a boot. It was an ugly one - reminiscent of what happened to Harry Newman (who suffered a broken leg last year).
“The tackle fell on him from the inside; fortunately there were no breaks in there - it is ankle ligament damage - but he is going to see a specialist over whether there’s a surgical requirement.
“Hopefully there’s not and it’ll be rest, recuperation and rehab’ for him.
“But there is a chance they may well look at surgery, depending on what the scan says. It is a blow, because the kid has been going great.”
Agar added that England prop Mikolaj Oledzki has a swollen foot following the Warrington game so they will assess him before deciding if he makes the 17 for tomorrow night’s home game with leaders Catalans Dragons.
Half-back Luke Gale (suspension), full-back Richie Myler (finger injury), back-rower Cameron Smith (hamstring) and prop King Vuniyayawa (concussion) are all back in contention and second-rower Sam Walters, who was not selected against Warrington, retains his place in the squad.
Prop Zane Tetevano has been sidelined with Covid but will not be risked due to a sore Achilles.