Lammens established himself as United's undisputed No. 1 during his debut Premier League campaign, keeping eight clean sheets in 32 matches after his £21.7 million move from Royal Antwerp.
Onana initially joined Manchester United from Inter Milan in 2023 for a package totaling £47.2 million.
However, his tenure at Old Trafford was marred by inconsistent performances, where he conceded 150 goals and kept 24 clean sheets across 102 appearances in all competitions.
Once the paperwork is fully processed, expected within the next 24 hours, Onana will join the Trabzonspor squad for pre-season training.
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Lammens is scheduled to return to Manchester United following his World Cup campaign with Belgium.
