I'd call him that yes. He played more minutes in league, CL and cup than Robben, Ribery, Mandzukic, Götze and started most of the big games (Dortmund away, City twice, United twice, Real once - and even against Arsenal, he had a huge impact from the bench, he wasn't frozen out of the team at any point). He started around 75% of all our games, if he's not a regular, who is?
There were always going to be rumours. Last season you could read about Robben's attitude being a problem and Pep wants him gone, Müller not fitting Guardiola's ideas, Thiago coming in meant that Kroos won't get enough minutes. It was all a load of made up nonsense and I doubt it's any different this time. Right now you can read everywhere that most of our players are having a shit season or don't fit into Pep's ideas. Only 4-5 weeks ago, every newspaper wrote that we're invincible, that the league will be boring for years, that all our players love the club and want to stay forever (with the exception of Kroos). It's just stupid. Müller signed a contract extension until 2017 last season, why should everything be totally different all of a sudden? We had a few bad weeks with a painful exit in the CL semifinals and now we have to live through the aftereffects. It'll calm down.