The Reds turned down a €100m (£90m) offer from Barcelona for Coutinho on 9 August and reiterated that the 25-year-old is not for sale.
However, Souness said Coutinho might have had his head turned by Barcelona's interest and believes that Liverpool boss Jurgen Klopp could ultimately be powerless to stop a transfer from going through.
The Catalan giants want to sign Coutinho as a replacement for Neymar, who joined Paris Saint-Germain in a world-record £198m move earlier this month.
"If they lose Philippe Coutinho, they'll get an enormous figure for him, but at this point of the window, they'll probably end up spending a lot of money on someone they are not sure about to replace him," Souness told Sky Sports.
"They have to fight to try and keep the player but ultimately the player will decide and for a Latin-American player, Barcelona will always be the place to go."
Coutinho scored 13 goals in 31 league appearances for Liverpool last season to help the club qualify for the Champions League.
His form was rewarded with a new long-term contract – without a buyout clause – worth around £150,000 per week in January, making him the highest-paid player at Anfield.
Souness said he could not see Liverpool winning the Premier League title or the Champions League this season, even if his former club manages to hold on to Coutinho.
"I don't think [Klopp's] made enough signings to challenge in the big two competitions next season," he stated.
"I think Klopp needs to make sure Liverpool challenge for the Premier League next season. I think that – alongside a run into the latter stages of the Champions League and one of the domestic cups – would be a good season for him."