Defending champion Matteo Berrettini has withdrawn from next week's championships at Queen's because of an abdominal injury.
A stomach muscle tear saw the Italian, 27, miss the clay-court season and last week he lost 6-1 6-2 to Lorenzo Sonego on his return to action in Stuttgart.
In 2021, the world number 21 won at Queen's before losing to Novak Djokovic in the Wimbledon final.
He beat Filip Krajinovic in last year's final to retain the Queen's title.
Berrettini withdrew from Wimbledon last year after testing positive for Covid-19 and now hopes to be fit in time for this year's tournament, which starts on 3 July.
Top seed and world number two Carlos Alcaraz will make his debut at Queen's this year and the 20-year-old Spaniard is due to face qualifier Arthur Fils in the opening round on Tuesday.
On the same day, unseeded record five-time champion Andy Murray is due to face a tough opening match against Australian world number 18 Alex de Minaur.
Fellow Briton Cameron Norrie, a Wimbledon semi-finalist last year and fifth seed at Queen's, will play Serbia's Miomir Kecmanovic on Monday.