The midfielder is expected to be out for several weeks as he recovers, and his involvement in the opening game of the Bundesliga season could be in doubt.
Not the news HSV needed
Hamburg would have been hoping to build upon a very solid mid-table finish last term with Holtby fully involved in pre-season.
However, the midfielder was posting photos of himself in a hospital bed watching on as his team-mates played in a friendly on Saturday.
The incident occurred on the return journey of a bike ride in the mountains. Holtby braked at the wrong moment and, consequently, fell off his bike and was taken to hospital.
HSV's worst fears were confirmed when the 25-year-old had to go under the knife to have the collarbone problem corrected.
Holtby remains hopeful
The diminutive midfielder was, arguably, the driving force behind the Volkspark outfit's improvement last term.
Holtby played in every single Bundesliga game, scoring three goals and setting up a further four.
"This is, of course, a great shock to us and I am very sorry for Lewis," said head coach, Bruno Labbadia.
After praising his attitude on the return to training, Labbadia added: "We wish him a speedy recovery and will support him as best we can!"
Does Demirbay's situation change?
With Holtby now a serious doubt for the start of the season, this could thrust Kerim Demirbay back into Labbadia's plans.
The midfielder, who has been subject to a €1.7m bid from Hoffenheim, has been told he can leave the club by Monday for €2m.
Demirbay has enjoyed two successful loan spells over the past two seasons, and would slot nicely into the role that Holtby plays.