Sebastian Vettel could link up with Lewis Hamilton at Mercedes next season after announcing that he will leave Ferrari when his contract expires at the end of the year.
Valtteri Bottas is not expected to be handed a new Mercedes contract for the 2021 campaign.
Silver Arrows team principal Toto Wolff must decide who to partner with six-time world champion Hamilton next year.
And he has admitted that Vettel is a viable option – although he would prefer to put faith in his young drivers such as George Russell or Esteban Ocon.
Horner was in charge of Vettel when the German won four Champions in a row with Red Bull from 2010-13 but he has ruled out a reunion with the 32-year-old.
However, he thinks it would be exciting if Mercedes went on to sign his former driver.
“Sebastian is a very deep thinker,” Horner said.
“He will have given this a huge amount of thought, I’ve got no doubt about that.
“What he’s achieved in his career is phenomenal, so if he were to decide that there’s not a competitive drive available to him at the end of this year, and he calls it a day… he could even take a year out if he wanted.
“Maybe Mercedes will fancy the headache of having him and Lewis together. It will be great for all of us to see and witness.
“From a team point of view, that would be a lot to take on.” concluded Horner.”
If Vettel is not offered a Mercedes seat he could decide to retire from F1.
There are not many other options for the four-time world champion, especially in teams challenging at the front of the grid.
“It’ll be a shame if Sebastian ends up leaving Formula 1, but he’s had an incredible career,” Horner previously said.
“He’s achieved huge, huge success and knowing him as I do, I’m sure he’s given this decision a huge amount of thought.
“He’s got great integrity, he’s got great morals and only he will know what he wants to do in the future.”