In 13/14 we had Aguero, Negredo and Dzeko. Jovetic was just a peripheral figure.
So we really had 3, maybe 3.5 strikers. And that was playing 2 up front.
Playing 1 up front we don't need more than Aguero and Nacho.
Even if we did, Pep always has his squads light up front. At Barcelona he only ever had 4 players for the front 3. At Bayern he had no back up for Lewandowksi except moving Müller up front.
I think it's most likely he will see it like this-
Aguero and Nacho are the strikers. If both of them are out of action, Sané is earmarked for being a future striker in Germany, he's got the height and strength to do it.
Sterling would be a viable 4th choice in an emergency, as would Silva who has played false 9 a lot for Spain, and Nolito has the movement and finishing to do it too.
Then there's kids in the academy to turn to.
With those options I doubt Pep will feel the need for a third striker.
And that's probably why we haven't been seriously linked with any.