The former Albion midfielder was forced off injured in Saturday’s 4-1 friendly win over Curacao.
Gilmour came off worst in a challenge late in the first half and after playing on for a few minutes he pulled up in clear distress and fell to the floor before after attempting a routine pass.
He was able to walk off the pitch and was immediately taken for a scan to determine if he will be able to fly out to America with the squad on Sunday.
Clarke said: “Obviously one big disappointment is the injury to Billy and we’re just waiting on it.
“He’ll be assessed and we’ll find out how bad it is. He’s gone for a scan so I haven’t had a chance to speak to him.
Asked if he was worried, Clarke said: “Yes, 100% worried. Obviously he hurt himself in the tackle and then he made the decision he had to come off so that tells you he himself knew it wasn’t right.
“We’ll just have to wait and see. There’s no point in speculating or thinking the worst. Let’s wait and see what the medics say. I’m sitting here hoping and praying that he’s on the plane with us.”
Gilmour moved to Napoli almost two years ago but remains popular with Albion fans.
He returned to Brighton to take part in a fund-rising sponsored walk for the BHAFC Foundation last summer.
If he is forced to withdraw, Tyler Fletcher, the 19-year-old son of former Scotland captain Darren Fletcher, has put himself in the frame for a dramatic call-up.
Fletcher got on for the last 16 minutes of Manchester United's 3-0 win at the Amex last Sunday having played the full 90 for the under-21s at the same venue eight days previously when Albion beat them 1-0 to win the Premier League 2 title.