Cosgrove was missing no fewer than seven players due to a fixture mix-up.
“We originally had no game this weekend, so some of the players took advantage and made other arrangements,” he said.
“Then we were told that we had a game but it was too late for the players to change so we had to play without them
“I was impressed with the young lads that we drafted in. Harlequins put us under pressure towards the end but David Glenny was good at the back and did a good job for us.
“Steven Young and Leigh Young also put in good performances.
“I also thought Owen Dooler in goals did well for us and made a couple of important saves.
“They could have sneaked one but the third goal from Jordan Robinson took the pressure of us.”
After only eight minutes Joel Cathcart went one on one with Ian Kelly but the Quins keeper blocked the danger only to see the ball rebound to Aaron Boyd, who saw his shot cleared off the line by Chris Gardner.
Two minutes later play swung to the other end and this time it was Dooler who pulled off a brilliant save from Chris Lemon.
In the 29th minute, Sam Campbell swung the ball from the right to find Phil Hamill loitering on the edge of the D and he scored.
Three minutes later, Mossley sprung open the homeside’s defence, this time Campbell unselfishly slipping the ball to Alan Monaghan to double the visitors lead.
Harlequins started the second half in search of an early goal, winning three penalty corners in the first 10 minutes but once again, Dooler was in brilliant form as he produced a couple of top drawer saves.
Harlequins just couldn’t convert their chances and as the clock ticked down, Mossley swung play to the other end for Robinson to drill home Mossley’s third.
Harlequins coach James Hutchinson said: “It wasn’t the start we wanted to the season and now with different teams in the competition we had been hoping to do well in it.
“We played well in the second half but we just couldn’t put away any of our penalty corners and it will be something we will be focusing on in training and hopefully we will get a better result in our next game.”