The former England hooker was left hospitalised after initially being diagnosed with bacterial meningitis, missing their crucial final Qualifiers game against Widnes Vikings as they eventually secured Super League safety.
It was later revealed, though, that he had actually been ill due to an abscess on his spine.
Lunt, 31, has been monitored closely since both by the hospital and by the Robins’ own medical staff, but is yet to resume training and is not expected to be fit for the Super League opener against Hull FC on February 1.
Asked when he hoped to have the former Leeds Rhinos star back in contention, Sheens admitted: “It’s hard to say.
“He’s had an update with the specialist last Monday and on the Tuesday he had a shunt – like a drip that he’d had in his arm – removed.
“They’d started off feeding antibiotics directly into his system since he got out of hospital so it’s been a long time. His body’s still reacting to that; he’s still not 100 per cent. There’s another scan to come so it’s still an ongoing issue.
“It’s one of those exotic things that I’ve not seen before, a bit like Danny McGuire last season with a blood clot in his calf.”
On his hooking options, the Australian added: “I’m working hard to get Tommy Lee up to speed – he’s our only other specialist. And we’ll make sure utility players Danny Addy and George Lawler are ready, too.”
Rovers have been linked with Australian half-back Josh Drinkwater, who helped Catalans Dragons win the Challenge Cup in August but is now a free agent.
Sheens said: “Everyone’s coming to Hull KR. Every time you don’t say ‘no’ to an agent and ‘I’ll have a think’ it’s out there that they’re signing.
“From our point of view, we never stop recruiting. At the moment I’m working with the squad I’ve got but what I’m really keen to do is get Shaun back on the field and Danny (McGuire) who has missed a week with a calf injury. It’s not a serious one.
“Lee Jewitt is coming back from his surgery and should be OK for round one but we know we won’t have Adam (Quinlan) for some time.”
Meanwhile, Red Star Belgrade are bringing one of the great names of European sport to the oldest competition in rugby league.
They are included in the draw for the First Round of the 2019 Challenge Cup on Friday December 14 at Emerald Headingley – the venue for the first Challenge Cup final in 1897.
The draw, which will be held before the second international between England Academy and Australia Schoolboys, will be streamed live on the BBC Sport website at 5.30pm.
Red Star, whose football club won the European Cup in 1991, have been playing Rugby League since 2006.
Their application to play in the Challenge Cup was accepted by the RFL on the basis they must play away in rounds one and two.