The Fulham midfielder has suffered ill-affects of a previous stress fracture and is set to miss Wales' 2010 World Cup qualifier in Azerbaijan on 6 June.
Frenchman Gilles Simon crushed 18-year-old American qualifier Donald Young 6-1 6-3 to set up a second-round meeting with world number one Roger Federer on Wednesday.
The 19-year-old served nine aces and hit 17 winners to set up a potential match with Roger Federer, who plays Indian qualifier Rohan Bopanna on Wednesday.
Second seed Gonzalez, seventh in the Masters Cup race, overcame Croatian qualifier Roko Karanusic 6-4 7-6 to set up a meeting with unseeded Finn Jarkko Nieminen.
World No. 46 Andreas Seppi beat his fellow Italian, qualifier Stephane Bohli, 6-1 7-5 to set up a second-round clash with either Serbian eighth seed Viktor Troicki or Austria's Jurgen Melzer.
Second seed Caroline Wozniacki came from behind to defeat Bulgarian qualifier Tsvetana Pironkova 3-6 6-3 6-4 and set up a third-round tie with Russia's Maria Kirilenko.
Ranked 920th, Santiago-born Garin beat Serbian qualifier Dusan Lajovic 6-3 6-4 on his tour-level debut to set up a clash with French third seed Jeremy Chardy.