Despite a late comeback at the Salford City Stadium, the Sharks failed to get past the league leaders as their survival hopes were dealt a blow.
"Harlequins disrupted the gameplan, they are fantastic at what they do so we've got no qualms and we were beaten by the better side," Diamond told Sky Sports News.
"Harlequins are a good side and we know they've got a good away record. We thought we could match that but obviously the gulf in the two sides was shown in the first half."
Friday's loss has left Sale one point from safety at the bottom of the table.