Defender Luke Chambers fears Nottingham Forest will struggle to match last season's run to the play-offs unless they can make some "quality" signings.
A total of nine players have left the squad during the close season.
And boss Steve McClaren has delayed joining the training camp in Portugal as he is worried by a lack of signings.
Chambers told BBC Radio Nottingham: "We've lost some very good players, and for us to challenge again, we need to get some decent quality in."
And Kris Boyd and Paul Konchesky have also not returned following the end of their respective loan spells.
There is also the threat that Guy Moussi could be on his way as he is out of contract and has not yet accepted the offer of a new deal.
Chambers, Forest's player of the season, added: "I know everyone's working hard behind the scenes but the longer it goes on, the more difficult it is to get a good start to the season."
McClaren has said the club must prove they are serious about challenging for automatic promotion by strengthening their squad.
"Other teams are showing their ambition," McClaren said. "We need to start showing ours because I didn't come here to not be competing for promotion.
"I want a good start and at the moment the biggest frustration is that we've not got the players in to work on what we want to do."