The three-time Formula 1 World Champion fell ill last summer whilst on holiday and had to have a lung transplant, leaving him unable to spectate any races in the second half last year.
But after contracting flu over Christmas, Lauda went back into intensive care in order to recover, pushing his return to the Mercedes paddock even further.
Speaking to Daily Mail Horner said: “Looking in from the outside, Niki’s absence is felt.
“Niki would be missed even more if it was a close fight for the title. He was there partly as a crutch for Toto [Wolff] to lean on.
“So the team will be weaker without Niki having a direct involvement for now.”
The Briton also went on to talk about the task current F1 owners Liberty Media and Chase Carey have of settling a new Concorde Agreement in 2021 for the benefit of the teams and the sport.
“Chase [Carey] has a task similar to Theresa May’s with Brexit: no deal will suit everyone, and the clock is ticking,” Horner said.
“Bernie [Ecclestone] was a dictator and that, in some respects, is what this sport needs.
“You never know with him. Maybe one day he will buy the sport back.
“He is still in good shape. He stays close to the teams, the promoters, governments.
“Liberty has addressed the areas where he was weak – future technologies and digital media, but in terms of negotiating a deal, there is no better person than Bernard Charles.”