The 26-year-old is expected to arrive in Greece on Monday to undergo a medical before completing the transfer.
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Dele-Bashiru has already agreed personal terms in principle and is set to sign a three-year contract once the move is finalised.
The Nigeria international spent last season on loan at Turkish club Genclerbirligi, where he made 28 league appearances.
Since joining Watford from Manchester City in 2019, he has scored six goals in 71 Championship matches.
Dele-Bashiru represented Nigeria at the 2019 FIFA U-20 World Cup after previously featuring for England at youth level.