The former Newcastle United and West Ham United striker played with Tosun at Besiktas during the 2014-15 season and during the second half of the 2017-18 campaign.
Tosun has joined Everton from Turkish giants Besiktas in the January transfer window for a fee reported by BBC Sport to be worth £27 million.
Ba, who spent the second half of last season on loan at the Istanbul club from Shenhua, has taken to Twitter to send a message to the 26-year-old forward.
The 32-year-old Senegalese striker has been on the books of Chinese club Shenhua since the summer of 2015 when he joined from Besiktas for a transfer fee reported by Sky Sports to be worth £11.3 million.
Everton have been crying out for a proven goalscorer since the departure of Romelu Lukaku to Premier League rivals Manchester United in the summer of 2017, and in Tosun they may have found an answer to their goalscoring problems.
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