But if the Leinster star regains full fitness in time for training on Wednesday, he has every chance of featuring among Sir Clive Woodward's Test 22.
Jonny Wilkinson, Gavin Henson, Charlie Hodgson, Andrew Sheridan, Ollie Smith and Ben Kay have already been been ruled out of Woodward's selection plans, while Horgan, captain Gareth Thomas, Gethin Jenkins, Neil Back and Will Greenwood are nursing various ailments.
Woodward abandoned training plans on Wednesday, but he remains upbeat of fielding a powerful Test squad, which should include prop Graham Rowntree, who was cleared at a disciplinary hearing after being cited for punching during the Auckland win.
Wilkinson's absence means that Welsh international Stephen Jones is favourite to line up at number 10, with Munster's Ronan O'Gara handed a bench role.
Fly-half Wilkinson's injury, another 'stinger' to his neck/shoulder area sustained during last Saturday's 48-18 second Test defeat, is the latest in a catalogue of setbacks since World Cup 2003.
He has only made three Test match appearances since then, and an initial neck and shoulder problem in early 2004 required surgery before causing an eight-month absence.
But Lions doctor James Robson insists that if the tourists had another game next week, then he would be available for selection, which will be music to Wilkinson's ears and those of his Newcastle boss Rob Andrew.
"If you question all the tour members, it is amazing how many people have had a stinger. It is a bruising to some of the nerves coming out of the neck and going into the shoulder," said Robson.
"It can take anything between a few moments or many months (to recover from). Jonny has had well-documented problems, and the other day he got two bangs on the same piece of nerve.
"The first one cleared very quickly in a few seconds, and the second one was of sufficient concern to actually bring him out of that danger area.
"Since then, he has made fantastic progress, and if this tour had two weeks to run, then I would have no doubt he would be available for selection next week. Jonny will recover very quickly.
"Gavin Henson has a similar problem, but will not recover in the next week or so. He may be several weeks, and his is taking longer to recover, although one would expect him to make a full recovery.
"If Jonny had a lasting problem, we simply wouldn't be able to get him fit enough to actually play.
"I didn't want him to have a third bang on the same place, because I know that each time you hit the same place, the damage is going to last longer."
Hodgson, who could well have taken over from Wilkinson in the Test team, suffered concussion after taking a heavy knock during the Auckland game, while centre Smith (ribs), prop Sheridan (ankle) and lock Kay (eye) also will not be considered.
Back (throat infection), skipper Thomas (virus) and centre Greenwood (shoulder) should all be available, but Robson admitted that Jenkins (ankle) was a concern, suggesting Rowntree's disciplinary clearance could prove timely.