The Royals manager must frantically scour the market for a centre-back after hearing that his captain will be moving to Championship opponents Coventry, who this afternoon defeated Reading 3-2.
Thirty-three-year-old Williams, who snubbed a potential move to Bristol City last week, underwent a medical examination at Highfield Road yesterday and is expected to be confirmed as a Sky Blues by the middle of next week.
The revelation is another bitter blow to Steve Coppell's ambitious side who were knocked out of the top two by their defeat at the hands of Adie Williams' future employers this afternoon.
Speaking after today's match, Coppell confirmed that he expects the deal to go through shortly.
"(The last time I spoke to Adie) he said he's agreed everything with Peter Reid and he was coming here (to Highfield Road) on Friday to have a medical," he said.
Meanwhile under-fire Coventry manager Peter Reid confirmed that he is hoping to conclude his negotiations with Adie Williams within the next few days.
"We're trying to strengthen and I've spoken to Adie but we're not very far down the line and these things can take time. I would like to have it sorted by the middle of next week but there is no guarantee we will get him," he admitted.
Steve Coppell decided to transfer list Williams earlier month when it was emerged that the veteran centre-back was unhappy with the club's decision not to open contract negotiations with him until February.
Now the Royals manager must find a replacement for the Welsh international to plug the gaping hole in Reading's back line.
Although make-shift central defensive partnership Ibrahima Sonko and Ivar Ingimarsson have impressed in recent weeks the duo shipped a disappointing three goals today.
The versatile Ricky Newman could cover at centre-back should either Sonko or Ingimarsson be forced to miss a match, but such a situation would hardly be ideal. Should two of Reading's three central defenders be ruled out disaster could await.
"In the short-term I'm ok, we've got Ricky Newman who has proven he can do a job there," added Coppell.
The Royals manager must quickly find a centre-back or he risks compromising Reading's impressive Championship standing. Steve Brown's decision to retire at the start of the season through injury increases the pressure on the Royals boss.