Onuachu is on a season-long loan move at Trabzonspor from Championship club Southampton.
The 29-year-old scored just three minutes into the game before Anastasios Bakasetas made it 2-0 in the 27th minute.
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Pendikspor pulled a goal back in the 36th minute through Endri Cekici but Trabzonspor held on for the win.
It is a return to winning ways for Trabzonspor who went into Saturdays game on the back of a 3-2 loss at Hatayspor on Monday, 25 September.
Trabzonspor move up to third place on 12 points after seven league games in the Turkish topflight.
Onuachu failed to score for Southampton in 11 Premier League games after joining them in January 2023.
He had scored 16 goals in 19 league games for Genk in the 2022/23 season Prior to joining Southampton.
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