It's a dangerous move--fans might remember who Joe Girardi doomed Josh Johnson to Tommy John with a similar move a few years ago.
Will Lannan get hurt? Who knows. But it's an unnecessary risk, and the manager's whole job is to play the percentages to optimize the outcomes from the team he has. To expose a key piece of his already weak rotation to injury is reckless.
But Riggleman will tell anyone who asks that he knows nothing about pitcher mechanics. And when asked about Kerry Wood, who he notoriously overworked as a 20-year-old rookie, he insists that there's nothing to learn from any pitcher who gets hurt--pitcher injuries according to Jim are entirely random events that can never be prevented by workload management, long toss, or anything else.