The 24-year-old is, however, finishing the campaign strongly with his hat-trick against City taking his tally to seven goals in his last six league appearances.
Koeman said the inconsistency was down to his youth.
"Sadio is a little bit unpredictable sometimes," the manager told British media. "He's still a young player, and young players they need sometimes to be more consistent.
"Sadio was focussed and clinical (on Sunday). That was one of my criticisms of all of the strikers but we are very productive at home with more chances to score more."
The win over City lifted Southampton above Liverpool to seventh in the Premier League, keeping Koeman's hopes of his team qualifying for the Europa League alive.
(Reporting by Simon Jennings in Bengaluru; editing by Sudipto Ganguly)
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