Southampton manager Russell Martin has described Portsmouth's promotion to the Championship as a "great achievement" for boss John Mousinho.
Martin revealed that the pair know each other well and played together at Under 18s level.
"I'm pleased a young British manager is getting an opportunity and I'm pleased for him personally," Martin said.
"For our fans though, it doesn't mean I'm really pleased that Portsmouth have been promoted."
The Blues confirmed their return to England's second tier for the first time in 12 years with a 3-2 win over Barnsley on Tuesday.
If Southampton fail to achieve promotion back to the Premier League this term, the two local rivals will face each other in the Championship next season.