Wolves have now gone three games without a win and have slipped to seventh in the Championship table.
"A few choice words were said in the dressing room," McCarthy told the club's official website following the 2-0 defeat at St Mary's.
"Southampton started well and we didn't, but as the game settled down we had as much possession as they did but we didn't hurt them.
"We can pass the ball about, but we can't afford to have a soft centre and give goals away.
"That's the first match, other than for 45 minutes at Barnsley, where we haven't created chances.
"This was probably billed as a much win game for Southampton and now Southend is a must win game for us. There's no dressing that up."