The Premier League champions have added Uruguayan defender Guillermo Varela to the squad, but activity has been quiet as of late, despite the club being linked with Everton's Leighton Baines, Barcelona's Thiago Alcantara, and Real Madrid star Cristiano Ronaldo.
"Don't be kidded by the lack of activity," Moyes told talkSPORT. "Take it from me, we are working hard behind the scenes to try and improve the squad.
"The big thing is that Manchester United will always be in for the best players in the world. We are also very interested in developing our own players and bringing in potential players for the future. We are working on that just now."
While striker Wayne Rooney has been linked with an Old Trafford exit, Moyes has been adamant that the England international will stay put.