"At 2-0 it could have been three or four and at that stage it looked like we would go on to win comfortably," the Saints boss said after the 2-2 draw at St Mary's.
"I'm sure Martin Allen thought he might be coming here and finding himself on the end of a hammering when we got our second goal.
"But their goal just before half time changed the whole complexion of the match and we started to get a bit edgy. We didn't play in the second half and in the end it could have gone either way.
"The Kevin Phillips goal was onside, I've seen the replay ten times and he was definitely on side.
"It was only a matter of inches and I cannot blame the linesman because he had to make a split-second decision.
"I took a chance today and played three up front because it was the FA Cup but I won't be doing that again."