Reports from Germany indicate that Vukcevic - who joined the Bundesliga club from Stuttgart's youth team in 2008 - was critical after undergoing an operation. He was freed from the smash on the road between Hoffenheim and Heidelberg on Friday.
"The firefighters had to cut the driver out of the wrecked vehicle. He was then taken away in a helicopter," said an eyewitness.
The truck driver escaped suffering only minor injuries.
"This news has shaken us," said Hoffenheim's general manager Andreas Muller. "The team, the coach and everybody here are shocked.
"Our thoughts are with Boris and his family."
Hoffenheim are due to play Augsburg in the Bundesliga on Saturday afternoon.
Vukcevic played in a 3-0 win over Stuttgart on Wednesday. He made his debut for the club against Schalke three years ago.