Bottas has impressed during his four years at Williams, finishing a career high of fourth in the 2014 World Championship and consistently outpacing the experienced Brazilian Felipe Massa.
But many fans were underwhelmed by the idea of the Finn taking the seat and had hoped to see Hamilton partnered by former rival Fernando Alonso.
One tweeted: "Hamilton will destroy Bottas. If they [Mercedes] are dominant then Hamilton will win everything."
Bottas is currently managed by Mercedes head of motorsport Toto Wolff, who would resign from the driver's three-man management team, if his move to Mercedes is finalised.
That move would likely see Massa postpone his retirement to drive alongside 18-year-old Lance Stroll.
The Williams driver was reduced to tears, after an emotional reception from his home fans in Brazil, during the second-last Grand Prix of the season.
Sauber's reported signing of Wehrlein paves the way for Mercedes to sign Valtteri Bottas, as Lewis Hamilton's team-mate for this season.
Merc bosses were weighing up their options, after Nico Rosberg sensationally quit the sport just five days after winning the world title. It left the world champions with a vacant seat for 2017.
However, with the majority of drivers tied in to long contracts, they were considering either youngster Wehrlein or William's Finnish driver.
However, with reports that Sauber have now agreed a loan move for the German, it would appear the road is clear for Bottas to partner Hamilton.
If the deal does go through as expected, it is likely Williams will recall Felipe Massa from retirement, as sponsors Martini require at least one of the team's drivers to be over 25.
Williams have also signed 18-year-old Lance Stroll.
