What decline last year? Are you talking about the ERA that nine of the 22 runs came from one outing in Milwaukee, and otherwise he had a 3.13 ERA in his other eight starts. Please don't say his win-loss record is a sign of decline, or the innings that the Cardinals didn't let him pitch out of concern for his longterm health. Let's look at other numbers that suggest some health concern:
His fastball averages (mph) the past three seasons -- 93.9, 93.6, 93.4
His slider averages (mph) the past three seasons -- 84.9, 84.5, 84.3
His curveball percentages the past three seasons -- 11.2, 12.1, 13.6.
He didn't change his mix of pitches favoring one more than another because of soreness or any indication of a decline in effectiveness that might be a signal to make some noise.
His expected Fielding Independent Pitching was actually better in 2020 than in 2019, and that number is based on what he could expect from the things he controlled -- swings and misses, balls, etc. His K/9 rate went up. His walk rate did, too, as he spent some time dialing in his fastball through the erratic schedule. Any sign of decline here? Not much signal there at all. If anything, the signals point again and again and again to a righthanded pitcher who four times was ready to make a start and had that start delayed -- by two days, by two weeks, by more than two months.
Subtract that Milwaukee start from his season -- the one where the Brewers clearly had some kind of insight on what he was about to pitch, through some way -- and check out his numbers from the quick back of the napkin calculations:
2.91 ERA, 10.59 K/9, 3.19 K/BB, and a 1.08 WHIP.
That's, um, pretty good.