Gerrard left the pitch midway through the first half of the tie against Lyon because of a recurrence of the groin injury he suffered while playing for England.
Winger Albert Riera needs a scan on a hamstring injury and teenager Martin Kelly, who deputised for Johnson at right-back against Lyon, picked up an ankle injury during the game.
Liverpool's Alberto Aquilani played the final 15 minutes oft he reserves' match against Sunderland on Wednesday as he gets closer to a first-team debut.
The Italian midfielder had knee surgery before his £20m move from Roma in August, but has been in full training for over a week.
Liverpool's Premier League title ambitions are already in a precarious state after four defeats in their opening nine games left them seven points behind Sunday's visitors and top-flight leaders United.
And Tuesday's Champions League defeat means the Reds are in danger of not qualifying from their group.
Lyon top the group with a 100% record from their first three games, while Fiorentina defeated Hungarian champions Debrecen to put them in the second qualification spot on six points.
Liverpool, who have three points from their first three games, visit Lyon on 4 November and then go to the Hungarian capital Budapest to take on Debrecen on 24 November before playing host to Fiorentina on 9 December.
Former Reds midfielder Dietmar Hamann, who is now involved with the club's academy, believes Benitez is the right man to lift Liverpool out of their slump.
He told BBC Sport: "Benitez is the right man. His record is excellent and he's proved he knows what is required.
"We have been in similar situations before and the fans need to support him."
Former Liverpool manager Roy Evans believes the Reds can still qualify for the last 16 in Europe's premier competition.
"When you lose at home in the Champions League it puts pressure on but a draw and two wins would probably see them through in second place," Evans told BBC Radio 5 live.
He also thinks the Liverpool hierarchy should stick with Benitez.
"Rafa has always been tactically strong in Europe but he's going to struggle without his best players," added Evans.
"And sometimes fans don't always agree with a manager's decision but you can't always blame the manager, sometimes the players have to look at themselves.
"Hopefully, Liverpool supporters, who are renowned for their knowledge of football, will get behind him and the club."
Meanwhile, BBC Radio 5 live summariser Pat Nevin believes a lack of structure behind the scenes at Anfield may prove to be Benitez's saving grace in the immediate future.
"One of the big things saving Rafa is that there is no great single control at Liverpool," Nevin told BBC Sport.
"If there was someone with their hand on the tiller his job may be under more threat but who is going to do it?
"Both of the owners aren't really talking to each other. They would have to come to some sort of agreement about that but they can't seem to agree about anything.
"The former chief executive, Rick Parry, was maybe the only person who was strong enough and he's gone."
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