Yes they will still be serious challengers. But my prediction is that Real Madrid will come back and be able to defend the title. This season Madrid are more solid because they have also brought in world-class players as needed. And the Ancelotti factor is also very influential.
I am not sure Real Madrid is still the favorite to win the title since they are not really serious to improve their squad. So far, they only bought 2 players, Rudiger and Tchouameni. These players aren't enough to improve Real Madrid squad quality, the need more new players, especially for CF position. Sadly, currently, Casemiro left the club, Real Madrid lost one of their key players in midfield line. Now, it depends on Tchouameni whether he can replace Casemiro role or not. If he failed, I doubt Real Madrid will defend the title.