Bottas has been out of sorts this year, finishing 37 seconds adrift of the leading duo in team-mate Lewis Hamilton and Red Bull's Max Verstappen in the season-opening race in Bahrain before crashing out in Imola after a collision with Williams driver and Mercedes junior Russell.
In Portimão, after qualifying on pole, Bottas was passed by Hamilton and Verstappen during the race en route to again finishing third.
A report in The Daily Mail, quoting an engineering insider, has suggested Mercedes is now losing faith and team principal Toto Wolff will replace Bottas with Russell at some point during the season if the Finn fails to improve.
Horner has had no qualms in the past in ditching drivers of his own during a season, notably bringing in Max Verstappen for Daniil Kvyat after just four races in 2016, and Alex Albon for Pierre Gasly during the summer break two years ago.
Horner, though, does not feel Wolff will follow his lead. In The Independent, Horner is quoted as saying: “Valtteri has demonstrated he can drive the car very quickly.
“Circumstances are always going to happen so let’s see how things pan out, but Valtteri has done a great job for Mercedes over the last few years.
“I would be surprised if they did switch Valtteri and George around mid-season.”
Despite Bottas' performances so far this season, for now, Wolff is publicly supporting his driver.
After the race on Sunday, Wolff said: “He will be on top form. He was in qualifying [in Imola].
"When you consider where he came from after Imola, that was a very write-off weekend and he put it on pole and had a very solid race, [and] he led."
Adding the team had let Bottas down due to a sensor issue that stopped his charge in hunting down Verstappen for second place, Wolff added: “All in all, like many times, it could have worked for Valtteri.
"We were pushing hard, we will continue to support him and next time we will see what he is capable of doing in Barcelona.”
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