The Sharks put in a tame performance as they crashed out of the Amlin Challenge Cup at the quarter-final stage.
Saints were 28-7 up at the break and kept their hosts at arm’s length for the majority of the second half to set up a semi-final at Franklin’s Gardens on April 25.
And Diamond, who had masterminded an Aviva Premiership win against Northampton less than two weeks earlier, was far from happy with his side’s display.
“We were well beaten,” he said. “There were inaccuracies in what we were doing, ill discipline and Northampton put us in those corners.
“We couldn’t defend the drive legally and conceded three tries from that.
“It was all over at half-time to be honest.
“We both had the same number of changes and it shows the depth of squad. A few people in our setup showed their true colours. We didn’t have a great performance by anyone.
“We were outenthused in the first half.
“We couldn’t have played any worse in the second half and it looks as though we did well then, but in reality it was very disappointing.
“They blew us away. Our scrum didn’t hold up. We’ve got international props who are not playing to the standard they do for the national team so we’ve got to have a look at our selection.
“We’ve got to dust ourselves down and go into the league next week.”