Premier League strugglers Cardiff paid a club record fee in the region of £15 million for the footballer, one of the leading scorers in the French Ligue 1.
Sala posted a photo to his Instagram and Twitter accounts on Monday afternoon posing with the rest of the Nantes squad.
He captioned the photo: “The final [heart emoji] goodbye @FCNantes.”
According to reports, Sala told his close friends he was 'scared' on 'a plane that seems like it is falling to pieces' in a voice note sent from the aircraft.
John Fitzgerald, chief officer of the Channel Islands Air Search, told the Associated Press he was not optimistic and added: “I don’t think the coastguard are either. We just don’t know how it disappeared.”
He said the plane “just completely vanished. There was no radio conversation”.
Guernsey Police said rescue workers searching the sea had spotted “floating objects” but could not confirm if these were from the missing aircraft.
The force tweeted: “During the course of the 15-hour search, which used multiple air and sea assets from the Channel Islands, UK and France, a number of floating objects have been seen in the water.
“We have been unable to confirm whether any of these are from the missing aircraft.”
Police said in a statement that the aircraft left Nantes at 1915 for Cardiff and was flying at 5,000 feet.
The statement added: “The aircraft on passing Guernsey requested descent. Jersey ATC (air traffic control) lost contact whilst it was flying at 2,300 feet.”
Amine Harit, a Moroccan footballer who played with Sala at Nantes before joining German side FC Schalke in 2017, suggested he was praying for the missing player in a tweet.
He wrote simply “Emi..” followed by a string of praying hands and crying emojis.
Nantes forward Randal Kolo-Muani retweeted Harit’s message on his personal account.
Sala’s former club, Chamois Niortais FC, has tweeted that the whole team is thinking of the missing player as the search continues.
“Tell us it’s a joke… Emi…. All the #TeamChamois thinking of you,” tweeted the clubs’s official account.