Ross Caldwell scored on his Morton debut in the 3-2 victory at Stenhousemuir.
The 21-year-old striker signed for the Greenock side on Friday, following his release by St Mirren.
Peter MacDonald, 34, has also been recruited by the League One side, and replaced Caldwell in the second half.
The former St Johnstone centre forward spent two seasons at Cappielow - scoring 27 goals - before moving to Dundee in 2013.
Manager Jim Duffy made it a hat-trick of signings by landing 23-year-old goalkeeper Grant Adam.