Keiron Cunningham was left to rue a series of defensive errors which cost Saints badly against a clinical Leeds side.
Saints did mount a spirited fightback but it was snuffed out by late tries from Liam Sutcliffe and Joel Moon as the champions crashed to a fourth defeat in five games.
And Cunningham admitted his side paid a heavy price for their first-half defensive errors.
“Our marker play was extremely poor, we became victims of our own errors in defence,” he said. “You can’t give as good a side as Leeds so many chances around the ruck. They’ve got a great nine and a lot of good support players.
“Resilience and scramble is always very good from Saints but our ruck defence let us down massively.
“The frustrating thing is that when we came out in the second half after speaking about it at half-time we turn into a different side. If we had done that in the first half, I am sure it would have been a completely different game.”
Cunningham defended his decision to reintroduce Walsh gradually following his eight-month injury absence, revealing that he will start next week’s game against Widnes.
“There was always a plan to introduce Luke Walsh back into the game,” Cunningham said. “He’s come from a very severe long-term injury and I had to hold my nerve.
“We’ll start him next week. I knew he’d make a difference in the game because he’s a great seven.”