The Football Association has blocked Southend's attempt to recall Andy Edwards from his loan spell at Grays.
Edwards joined Southend from Rushden & Diamonds in 2004
The Shrimpers wanted the 34-year-old defender back after Spencer Prior and Efe Sodje were ruled out for the rest of the season by injury.
With Southend closing on promotion, boss Steve Tilson told BBC Essex: "It's a setback, but we've had other setbacks this season and got over them.
"We've just got to get on with it now. It could be a special weekend for us."
Tilson's side face Gillingham on Friday and Bradford on Easter Monday - and could seal promotion with three games to spare if other results go in their favour.
The Shrimpers have handed one-year professional contracts to two youth products - winger Joel Ledgester and midfielder Callum Kearns.
They have also offered a one-year deal to trialist David Cowley, a former West Ham winger.