The Hornets boss has hinted that the former Aston Villa and Reading full-back may well have done enough to impress him over the past ten days to earn himself a contract at Vicarage Road.
"Ulises has also done well for us, it may be that he's earned himself a contract, we'll have to wait and see.
"He's only been with us for a week and a half but he's enthusiastic and he's just what I want on the pitch," Boothroyd has said.