When's the last time a 3rd/4th line grinder made his way on a top-6 at 26 years old? He might have a bit more speed and vision than your usual grinder but i can't see him getting a regular spot outside the bottom 6, even a regular third line spot seems unlikely.
He would have made a fantastic RWer next to Doug Jarvis and Bob Gainey.
Definitely can handle 3rd line RW. It makes sense to play him with Desharnais and Pacioretty at times on the third line, and Patches can get extra minutes on the 1st PP unit with Pleks and Galchy!
Galchenyuk-Plekanec-Parenteau
Sekac-Eller-Gallagher
Pacioretty-Desharnais-Weise
Bournival-Malhotra-Prust
If Gallagher slumps a bit from being over-used, drop him to the third line, and put either Weise or Prust with Eller. Also Sekac and Pacioretty can switch up where needed.
Other than Galchenyuk being married to Plekanec in my books, playing the top minutes and being on the first PP wave, pretty much all the wingers are interchangeable. Just call Eller's line the 2nd line, Desharnais' line the 3rd line and Malhotra's line the 4th line. In real life, use them according to situations and who is playing better.