The six-times champion looked set to be without a ride as his retained trainer Luca Cumani does not have a runner, while he was unlikely to pick up another spare ride for former boss Aidan O'Brien.
Fallon rode one of the Ballydoyle outsiders, Viscount Nelson, in the 2000 Guineas earlier this year but with O'Brien's options decreasing, Godolphin have swooped for his services. Al Zir finished ninth in the Guineas and, with Frankie Dettori aboard first-choice Rewilding and Ahmed Ajtebi on the surprise supplementary entry Buzzword, the Racing Post Trophy third will have Fallon's assistance in the saddle.
"Kieren Fallon's record in the Derby is excellent," Crisford told www.godolphin.com. "He was available and has ridden for Godolphin before."
Al Zir finished third in last year's Racing Post Trophy and is as big as 33-1 for Derby glory. Fallon has won the Derby three times on Oath, Kris Kin and North Light.
Jan Vermeer has hardened as favourite for Saturday's Classic after St Nicholas Abbey, his stablemate and long-time market leader, was ruled out of the race.
The impressive Gallinule Stakes winner is 13-8 with totesport, having leapt to the head of the betting at around 2-1 even before the announcement.
St Nicholas Abbey had not shown his usual sparkle in a workout on Friday and yesterday O'Brien revealed the colt had suffered a slight muscle injury in a hind quarter.
"The vets found a slight muscle injury high up in his hind quarter. This meant he could not work today and we were advised to give him an easy few days. Obviously this will preclude him from running in the Investec Derby on Saturday," said the Ballydoyle trainer.
"We always thought St Nicholas Abbey could walk on water and it is very unfortunate for all concerned to have a setback like this so close to the big race."
O'Brien still has four other entries including Cape Blanco, but Coral have removed the Dante winner from the betting as he is tipped to head to Chantilly for the French Derby on Sunday.
Investec Derby. Coral bet: 7-4 Jan Vermeer, 4-1 Workforce, 6-1 Midas Touch, 7-1 Bullet Train, 8-1 Rewilding, 11-1 Azmeel, 16-1 Coordinated Cut, 20-1 Al Zir, 33-1 Ted Spread, 50-1 bar.