Northampton's director of rugby told BBC Sport: "It was a poor performance. In the first half we never really got off the coach. Sale scored some good tries and took their opportunities. In the second half we improved but we were always up against it because we knew we had to score tries.
"We looked like a side who didn't have any cohesiveness out there because we haven't all played together for eight weeks. I don't think it is fitness because our lads are fit. We just couldn't string anything together.
"We've got some important games coming up now. We've lost this game but we're not out, we've got five games and we're still up in a very strong position."
The defeat was the team's first since their loss to Gloucester in September.