Charlton striker James Walker's £200,000 move to Southend fell through because the 20-year-old was found to have an irregular rhythm of the heart.
Walker scored three goals in 14 appearances while on loan at Yeovil
BBC Essex says Walker, who had signed a two-and-a-half-year deal, told the Essex club previous medical tests had revealed a similar problem.
But a consultant at Southend Hospital told chairman Ron Martin his condition should be monitored for three months.
In the meantime, he should cease all football and training.
Southend had announced the signing of Walker earlier in the week.
Meanwhile, Plymouth striker Rory Fallon, 25, has rejected a proposed £250,000 move to Roots Hall.
The New Zealander, who joined Plymouth from Swansea nearly a year ago, wants to fight for his place at Home Park.