The Ibrox gaffer had hoped to bring in up to five players before the club's year-long transfer embargo kicked in on Saturday.
But despite David Templeton arriving last week, Coisty is concerned about the season ahead.
He said: "I'm not happy. I said last week that I wanted to get four or five bodies in and we got one or two in and one went out.
"I know we are thin on the ground in defensive areas. We will have to carry a bit of luck in terms of injuries and suspensions."
The Gers gaffer is waiting for international clearance for Francesco Stella, who has previously played for Italian Serie A club Siena. But with captain and centre-half Carlos Bocanegra joining Racing Santander on loan for a season, his squad is thin.
And he will now be unable to buy a player aged over 18 until September 2013 due to the 12 month embargo.
McCoist, who saw new boy Templeton net a debut brace yesterday, said: "It just didn't happen for us. There was no shortage of players we were interested in.
"Neil Murray, our chief scout, has been great. Kenny [McDowall], Ian [Durrant] and myself had a list of players we wanted.
"Unfortunately, it just hasn't happened.
"It was a combination of a lot of things, but there is no use going on about it now, we just have to get on with it. At least we know where we are."
He added: "Francesco Stella has come over from Italy and he is a player we have decided to give a short-term contract to. Hopefully, he will do well.
"I'm not sure if international clearance has come through. Andrew [Dickson, Head of Football Administration] and his boys will be checking on that and hopefully they will get it through."