No other recent Lufthansa CEO has forced so much deep change on the airline as Christoph Franz. The announcement of his unexpected departure creates uncertainty, drawing into question whether Europe's largest airline group will continue with reform or stagnate.

Franz confirmed last week that he will not extend his contract, set to expire next spring. Instead, he will become executive chairman of the board of Swiss pharmaceutical giant Roche, replacing Franz Humer. Franz has served on Roche's board since 2011.

The CEO's departure comes at a time when the company is still struggling to adapt. Franz has been the mastermind behind Lufthansa's most fundamental relaunch, which is far from complete.

When he was leading the group's passenger business, Franz instituted the “Score” restructuring program aimed at cutting costs by €1.5 billion ($2 billion) starting in 2015. Score includes 3,500 job cuts, unheard of at the consensus-oriented airline, and hundreds of projects to improve efficiency. The largest undertaking is the transfer of non-hub European flying to low-cost affiliate Germanwings, a decision made after Franz recognized that Lufthansa would never be able to compete effectively outside of its hubs. The move remains highly controversial, even among senior management and on the executive board level.

But Franz has not only initiated cost cuts; he has also been the first Lufthansa CEO to open the company to outside expertise. For decades, the airline was inward-focused in its search for talent and ideas. That has changed so radically that now long tenure with the company is almost a disadvantage for career progression.

It is no secret or surprise that Franz is disliked by many. Unions representing pilots, cabin crew and ground staff are quietly expressing hopes that his successor will slow things down, although Franz is stressing that there is “no alternative” to the current course, which he sees as a prerequisite for future investment.

And the investment will be huge. The airline last week placed orders for 59 new widebody aircraft, including 34 of the yet-to-be-launched Boeing 777-9X and 25 Airbus A350-900s. That alone represents a €14 billion investment at list prices; the carrier has 295 firm orders in total. Even with the usual discounts, Lufthansa is likely to spend in excess of €20 billion for new aircraft.

“Without the successful implementation of the restructuring program, we will not be able to generate the necessary means to finance the investments,” Franz says. “The orders are an expression of our confidence that we can implement Score.”

A successor has not been named, and company officials indicate a decision will not be made for several weeks—possibly as late as year-end. Franz will stay on until May 2014. The board has made clear that it intends to continue the restructuring course.

Many see Carsten Spohr, 46, as the leading candidate to replace Franz. Spohr began as a Lufthansa pilot and is an Airbus A330/A340 captain, but since moving into management, he has kept his license current only in the simulator. He followed Franz at the helm of the airline operation, the division most in need of restructuring, and is well-liked among staff. But Spohr has not pushed for reform as hard as Franz, and critics say he has not proven that he is up to being CEO. Other units such as Lufthansa Cargo and Lufthansa Technik are already delivering promising results.

Another contender is Harry Hohmeister, currently CEO of Swiss International Air Lines and group executive board member overseeing Austrian and Brussels Airlines. However, he was promoted on the group level only in July, raising doubts about his readiness to step into a still larger role.

Lufthansa executives say the board may appoint an interim CEO who would serve for a few years in order to give themselves more time to search for a permanent replacement. Lufthansa Cargo CEO Karl-Ulrich Garnadt, 56, would be the most likely candidate in that scenario.