Saifullah joined Young Lions after his national service and played the out the remainder of the 2018 campaign. While on trial at Spanish club, CD Tenerife, he did enough to be offered a contract with the club's B team, however he chose to remain with Young Lions for the 2019 Singapore Premier League season due to developmental reasons.[15] Saifullah became a key member of the Young Lions squad that season, featuring regularly on the wing under Fandi Ahmad.
On 17 May 2021, Saifullah was called up to the Singaporean national team for the Asian World Cup qualifiers in June.[16] On 3 June, he made his senior debut in a defeat to Palestine, coming on as a substitute for Hafiz Nor in the 65th minute.[17]