The AlphaTauri driver suffered a metacarpal fracture in practice for last month's Dutch Grand Prix, ruling him out of the Italian Grand Prix one week later as the Australian underwent surgery in Barcelona.
Horner, whose team has loaned out third driver Ricciardo to sister squad AlphaTauri, says Ricciardo's recovery is going to plan but he believes the next double header on the calendar in Singapore and Japan is going to come too soon for the 34-year-old.
“Certainly Singapore, I don't think there's any chance he'll be ready for then. I think it would be optimistic for Japan," Horner said when asked for a timeline of Ricciardo's return.
"But I think his recovery is going well, he's got obviously immobility of the hand and he's into rehabilitation now.
"We’ve seen with motorcyclists rushing comebacks can sometimes do more damage, so I think that we just want to make sure he's fully fit before he gets back in the car.”
In Zandvoort, Red Bull called on its junior driver Liam Lawson to replace Ricciardo from FP3 onwards. In Monza, the 21-year-old New Zealander completed his first full grand prix weekend, finishing just outside the points in 11th.
There was no way of comparing his performance to team-mate Yuki Tsunoda, who failed to make the start due to a power unit issue on the formation lap.
“I think he did a solid job today," Horner replied when asked by Motorsport.com for his verdict on Lawson's performance.
"I haven't followed his race that closely. They went for a two stop, which was unconventional on today's strategy.
"So, I'll have to have a look at the analysis after the race, but I think he's acquitted himself very well.”
While deputising halfway through the Dutch GP weekend was a huge challenge for Lawson, and the tricky streets of Singapore will be equally daunting, the Kiwi knows Suzuka well as a Super Formula frontrunner in Japan.
“I should think he's pretty keen to be in the car in Suzuka, but we'll just take it on a day-by-day basis and see how the recovery and nature takes its course," Horner added.
Lawson similarly refused to look too far into the future while Ricciardo recovers from his injury.
When asked if he was excited at the prospect of trading his Super Formula car for an F1 car at Suzuka, he replied: "If, exactly. If I get the chance.
"I have no idea what's going to happen right now. So, I'll prepare like normal, but we'll wait and see."
Additional reporting by Adam Cooper
