Saints blew a 2-0 lead to lose 2-3 at St Mary's.
Martin insisted afterwards: "I was really happy with a lot of the performance and a lot of the performances of the team.
"Mateus was one foul away from getting red-carded as well because he'd had a booking and made a few and was a bit fatigued from his international travels.
"I will look at the change we made and all that stuff but I think genuinely the red-card has really changed the game for us.
"I think it was a brilliant time for us to get through a difficult moment when we conceded 2-1 and felt we did and then that happened."
On Will Smallbone's injury, Martin added: "It looks like he's tweaked his hamstring again which is really disappointing for him and for us.
"He soldiered on and battled through the last 10-15 minutes with that for the team because that's the kind of guy he is."