Over the last 4 years, Shields has averaged 233 IP, a 3.17 ERA and 1.15 WHIP in the AL. That's 1st in IP, 5th for ERA, 6th in WAR, 5th in Wins, 8th in WHIP over that span.
Most of those numbers are better than Verlander's, Darvish's, Scherzer's and Lester's. Very similar to Price/Felix, except those guys had slightly better ERA/WHIP over fewer innings. He has been worlds better than real #2s like Anibel Sanchez, Josh Beckett, John Lackey.
Shields is a #1 pitcher. That shouldn't be questioned. The question is whether he will still be an ace into his mid and late 30s.