The Belgian rider was drinking from a bidon about 50km from the finish when he hit a pot-hole in the road, going over the handlebars.
He suffered severe facial trauma in the crash, with several fractures. He also has concussion, three broken ribs and has bleeding in his lung.
Boeckmans was taken to hospital in Murcia. Lotto-Soudal say that he is expected to be kept in a coma for a few days, and will "probably need surgery".
The crash had knock-on effects in the peloton, causing a number of other withdrawals. Tejay van Garderen (BMC Racing), who was expected to challenge for the overall general classification, was forced to retire from the race with a broken collarbone. Dan Martin (Cannondale-Garmin) and Nacer Bouhanni (Confidis) also had to withdraw.
"We are all thinking of Kris tonight," said Boeckmans's team-mate Tosh Van Der Sande. "I was riding just behind him when it happened. He was drinking when he rode over a hole, tumbled over his handlebar and hit the ground very hard."
Thomas De Gendt, also of Lotto-Soudal, wrote on Twitter that he was "crying like a baby" to see Boeckmans so badly hurt.
Numerous other riders took to Twitter to wish Boeckmans well.
Stage eight was won by Jasper Stuyven (Trek Factory Racing). Esteban Chaves (Orica-GreenEdge) holds the overall lead.
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