Sebastian Vettel grew accustomed to winning in his years at Red Bull. Things are a little different now that he’s at Ferrari, with Mercedes winning race after race. And that frustration could lead him to jump ship.

That’s not our opinion, but that of Christian Horner. And he should know, having employed Vettel for six years and orchestrated his four back-to-back world championships at Red Bull. Horner told German publication Sport Bild that, if things don’t change at Ferrari, he could see Vettel seeking a new contract with Mercedes instead.

Vettel made his race debut for BMW Sauber in 2007 before moving to Scuderia Toro Rosso. He continued with STR the following season, taking the team’s only race win to date at the rain-soaked 2008 Italian Grand Prix. The following season he moved up to Red Bull, winning four more grands prix before kicking off a string of four back-to-back world championships.

Once the current formula came into effect in 2014, Mercedes took Red Bull’s place as the dominant team, and Vettel didn’t win a single race that season. He moved to Ferrari the following year, winning three races to land third in the championship standings. This year he’s landed on the podium in five out of ten races campaigned so far, but has yet to take a checkered flag home this season.

Meanwhile Mercedes has won all but one race this season, with Nico Rosberg and Lewis Hamilton trading places atop the podium – and only Vettel’s successor Max Verstappen taking a solitary win for Red Bull. Vettel has been outperformed by Kimi Raikkonen so far this season as Ferrari boss Sergio Marchionne puts the pressure to his star drivers to win races.

Ultimately that pressure, says Horner, could be what drives Vettel to move elsewhere. With Mercedes proving the force to be reckoned with in F1 today, he could very well follow his onetime mentor and countryman Michael Schumacher to leave Italy for his native Germany.