Coutinho missed the Reds' final pre-season friendly of the summer last weekend - a 3-1 win over Athletic Bilbao - and has not trained with the rest of his teammates for the past week as a result of the niggling issue.
"No, he's not available [for Watford]. He's not been in training since last Friday. He's also a doubt for Tuesday unfortunately," he told reporters.
Asked to further comment on the ongoing speculation, he added: "When FSG say something like this it's 100% clear. You can read it. We want to have the best possible team. This means keeping the guys we have.
"Liverpool is not a club that needs to sell. That is set in stone. So what they pay in the end doesn't matter. From a financial standpoint, there's no threshold of pain."
Former Inter Milan ace Coutinho played a direct part in 20 goals in 31 Premier League appearances last time out.