Ghanaian youngster Majeed Ashimeru provided an assist but it was enough as St Gallen were beaten at home by FC Basel in Swiss Super League on Saturday.
The former WAFA star found Sierro with a delightful pass to open the scoring for Gallen but Basel fought back and deservedly won the match 3-1.
He played the full throttle of the game but the defeat leaves the Espen in 5th position after 10 matchdays.
Ashimeru has featured in all of Gallen's games this season, impressing greatly.