The 28-year-old defender played the full 90 minutes and scored a solo goal as Cherries came from behind to defeat Derby County 2-1 in Dorset.
In order to build up his fitness, Bournemouth wanted to use him in the opening match of the Premier League Cup youth competition alongside their development side.
They had to seek permission from Saints to do so, while Stephens agreed to play happily and was credited for 'setting an example' by coach Shaun Cooper.
Cooper said: “I mean, there's no surprises. We know he is a quality player, he came in and he was just that. But the best thing about Jack is his attitude, his professionalism.
“To want to play was the first thing, but then to handle himself as professionally as he did and set the example, and guide and communicate to the rest of the players throughout the game."