Kenny Rogers. The free-agent-to-be was 5-3 with a 4.30 ERA as a member of the Oakland Athletics at the time of the trade. His final 12 games as a member of the Mets were a smidge more productive. In those dozen starts, Rogers went 5-1 with a 4.03 ERA.
If you remember baseball back in 1999, hitters were dominating the game. This was peak Steroid Era. The Mets starting staff was unable to escape the onslaught.
The 1999 Mets won 97 regular season games and among those who started 10+ times, Rogers had the lowest ERA.