|Date of birth||January 31, 1989|
|Place of birth||Bwiam, Gambia|
|Height||5 ft 9 in (1.75 m)|
|2001–2004||Gambia Ports Authority|
|2004–2007||Gambia Ports Authority||82||(18)|
|2007–2013||New England Revolution||104||(8)|
|* Senior club appearances and goals counted for the domestic league only and correct as of October 28, 2017
‡ National team caps and goals correct as of 12 May 2017
Nyassi was scouted by New England Revolution head coach Steve Nicol while playing in the 2007 FIFA U-20 World Cup, and signed with the Revolution shortly thereafter along with his Gambian teammate, Kenny Mansally.
He made his MLS debut on September 9, 2007 against D.C. United as a substitute, playing the final eight minutes on the right wing. He made his first start and scored his first MLS goal on March 29, 2008 in the Revs' 3-0 opening day victory over Houston Dynamo.
Nyassi signed with FC Edmonton in February 2015. Nyassi would spend three seasons in Edmonton. After the 2017 season, with the future of FC Edmonton and the NASL in doubt, Nyassi was released from FC Edmonton. 
Nyassi has represented his nation at various youth levels. He played for the Gambian U-17 national team in the 2005 U-17 world championships, and for the U-20 national team in the 2007 U-20 FIFA Championships. He scored his first goal for the Senior national team on September 4, 2010 in the 2012 Africa Cup of Nations qualification match against Namibia.
- Scores and results list Gambia's goal tally first.
|1||9 January 2010||Stade El Menzah, Tunis||Tunisia||
|2||4 September 2010||Independence Stadium, Bakau||Namibia||
||2012 Africa Cup of Nations qualification|
- "Archived copy". Archived from the original on July 14, 2014. Retrieved June 12, 2014.
- "Nyassi Set to Sign with the Eddies". February 10, 2015.
- "FC Edmonton announce four departures in Ameobi, Corea, Fordyce and Nyassi". FC Edmonton. November 16, 2017.
- Steven Sandor (November 16, 2017). "With the club's path unclear, FC Edmonton begins releasing players". the11.ca.
- "Nyassi, Sainey". National Football Teams. Retrieved 12 May 2017.