The Saints were beaten 1-0 at Norwich City on Saturday afternoon, continuing their poor recent run.
The Saints were once again beaten at Carrow Road to make it just one win in eight for the South Coast side. Fascinatingly, that one win was a 4-0 triumph against Arsenal.
But that was very much the outlier when it comes to Southampton's recent results.
Koeman's men are now done to 13th in the Premier League table, seven points off the relegation zone. And the Dutchman focused in particular on his team's lack of discipline on Twitter after the game:
I tried to explain why I am angry after this defeat! This is not the discipline what we like in the Club to be successful!— Ronald Koeman (@RonaldKoeman) January 2, 2016
Koeman dropped Mane because he missed a pre-match meeting, something he told BBC Sport was 'unacceptable.'
And Sinclair stopped short of defending the forward but did suggest he could be leaving St Mary's in the future.
Next up for Southampton is an FA Cup clash against Crystal Palace, before a Premier League clash against Watford. Neither will be easy - and both will require far better from the South Coast outfit.