Nick Williams out of Saracens battle but Jared Payne and Darren Cave nearing a return for Ulster
Ulster are resigned to being without Nick Williams for their Champions Cup trip to Saracens on Saturday (3.15pm kick-off).
The burly No.8, who will leave the province for Cardiff in the summer, was concussed in the final minutes of Sunday's dramatic fightback against Oyonnax at the weekend.
Currently undergoing the return to play protocols, the six-day turnaround gave him little chance of recovery.
Sam Arnold, another who was injured at the Stade Charles Mathon, will be absent for a period in the region of two weeks with a hamstring strain.
Wiehahn Herbst and Alan O'Connor are pushing for a return in the pack but the composition of Les Kiss' backline for the trip to London is less clear.
Andrew Trimble is due to return but there remains doubts over Louis Ludik's quad injury.
Jared Payne and Darren Cave are, with just a week remaining before Joe Schmidt is expected to name his initial Six Nations squad, still attempting to prove their fitness.
Ireland kick off their title defence against Wales in the Aviva on February 7 but Kiss says that he, rather than the IRFU player management programme, will have the final say over his selections.
Having spared Paddy Jackson the start against Oyonnax, while Trimble was left at home, the rest of those who aim to be involved in Schmidt's plans do not necessarily have to be handed a rest over the next two weeks.
Kiss said: "We've our own player welfare and we're trying to use that as well but just to be smart about how we utilise those.
"It'll be our selections and we'll select to win."
Having taken four, but not the full five, points available from Oyonnax at the weekend, Ulster remain very much in the hunt for a quarter-final spot.
While they will appreciate the requirement to get a bonus-point win in the return game against the French strugglers next weekend, Kiss says his side need to take something from Allianz Park on Saturday rather than rely on help from the other pools.
"We have a chance, without a doubt," said the Australian.
"It's a massive week for us and not an ideal one.
"Saracens got beat (last week) so they'll be smarting.
"It doesn't get any easier for us but we're in the hunt and that's the main thing. We need to get points out of this week somehow and we need to deliver a lot better at home against Oyonnax in the next match.
"At least there's a chance for us."
Former Ulster head coach Mark McCall, now in charge of Saracens, is sure his table-topping Premiership outfit are ready to bounce back after suffering what was their first defeat in any competition at the weekend.
"This group in the past has been pretty good at dealing with the odd setback," said the 2006 Celtic League winning coach.
"We've got a big game to get our teeth stuck into against Ulster and we'll get stuck into it this week."
Flanker Will Fraser, who scored a try when the teams met at the quarter-final stage in 2013, echoed that sentiment but is sure that Ulster will have revenge on their minds after earlier this season seeing Saracens become the first visiting side to secure a bonus-point European win at Kingspan Stadium since 2007
"Ulster is a huge game," said the 26-year-old. "We had a good win against them away so they're going to be fired up for the game at Allianz Park and so are we."