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.
I'm the first to admit that, as a normal fan, the real information on such matters one can get is extremely limited, so all we can do is speculate.
Nevertheless i follow the transfer antiques for a couple of decades now. It was quite clear that some of those rumours - you named the Robben one - were more a feeble attempt of some newspapers to fill empty mid-week sports pages than anything else.
The Müller rumours have a different scent to them, so i'm not that confident.
Yes, he played a lot, but he was only subbed in in both Arsenal and the first Real leg, and in the Buli match against BVB. The most important matches of the season.
Now Müller is not one to openly complain to the media like some like to do, but he isn't stupid and we can be sure there *exist* certain pundits that have a good access to what he really thinks.
After every one of those matches there were remarks in the newspaper reports that he isn't happy at all about being second choice when it really matters.
He also isn't that commited to playing for Bayern alone, never was. Remarks like the one Piratesoup quotes here have been in his interviews for years.
New Müller interview. States that he's not about to leave at all, but that he also can't promise that he'll stay forever.
http://www.goal.com/de/news/827/bundesliga/2014/05/06/4798905/bayerns-thomas-müller-stehe-sicher-nicht-vor-dem-absprung
Both him and his wife are cosmopolitic and open-minded enough to see living and playing in a different country or culture as an exciting thing to do rather than just second choice to building a nest on the Tegernsee, not as much as, for example, Ribery, Schweinsteiger or Lahm, who quite openly made their choice public.
That is why i rate his chances leaving higher than those of many others, including Kroos, who might be a bit scared about his chances to succeed abroad. I don't think Müller is in a state of "i'm so pissed i'll be out in the second a good offer reaches me", but i believe he has a certain plan of the conditions he wants to play it, and if those conditions are not present, and the package offered to him are to his liking, he will leave without regrets.