McLaren has confirmed that Stoffel Vandoorne is on standby to drive in the Chinese GP, pending an FIA medical check for Fernando Alonso on Thursday morning.
Vandoorne travelled to Shanghai on Tuesday, on the same plane as team boss Eric Boullier.
“Once again, Stoffel will be on standby until Fernando has his routine meeting with FIA doctors on Thursday,” said Boullier. “And until then we will be readying ourselves as normal. Fernando has been recuperating at home and training as usual, and we, like him, hope to see him back in the car. We’ll accept the outcome – whatever that may be – and plan accordingly.”
Meanwhile Alonso said: “It was disappointing to be told I couldn’t race in Bahrain, but I fully respected the decision of the FIA medical team. While I hope I’ll be back in the cockpit on Friday, until I get the all-clear from the doctors to race – whenever that may be – we cannot assume anything, but I’m continuing to prepare for the race weekend as normal.”
Adam Cooper's F1 Blog 
How long does a cracked rib take to heal? I’ll say more than a week or two, and that is just a wild guess.
This may turn into a Wally Pip-Lou Gherig situation.
American baseball reference: After an injury, Lou Gherig replaced Wally Pip and poor Wally never saw another start, Lou was that good.