A year out from the launch of the 125th anniversary tour test series, Horan pointed to the inability of David Pocock 's side to put Wales away in the second Test in Melbourne as a major concern.
"Had Mike Harris' penalty kick drifted a metre to the right, we would be re-evaluating the match as a poor performance by the Wallabies," Horan wrote in the Daily Telegraph in Sydney.
"I praise the team for the poise and leadership they showed to grind out a victory in the end, but the statistics show they should have won far more comfortably. Wales missed 29 tackles to 11, conceded 12 penalties to seven and had just 49 rucks to Australia's 114.
"The difference in running metres between the sides was astounding, Australia's 817 to Wales' 329. And it came down to a kick after the bell.
"For the Wallabies to fire against the Lions they'll need players like Wycliff Palu to stand up. He has been quiet in the past two Tests and to beat the Lions you need your big forwards to grab games by the scruff of the neck.
"The Lions will field one of their most inexperienced line-ups in terms of Test caps when they tour here next year, but they will be one of the most talent-laden outfits in the history of the Lions."
Horan played twice against the Lions on their 1989 tour when he was launching his career, but missed the 2001 trip after switching to play at Saracens. The Aussies' won the 2001 series 2-1 and Horan is worried the outcome might be different in 12 months time.
"Wales and England have a number of young stars developing rapidly on the international stage and for the first time the Wallabies will have to prepare immediately on the back of Super Rugby," added Horan.
"As we've seen this year with the Scotland result, you need adequate time to prepare. Coming in for internationals on the back of more than three months Super Rugby will be extremely challenging.
"Test rugby is a big step up from Super Rugby, but playing the Lions is tougher still. Depth has always been a strong suit of the Lions. You merely have to look at the centre spots, where they will choose from Jamie Roberts, Brian O'Driscoll and Manu Tuilagi."