The stats seem to point to one main issue for the Rockies slugger.
The most noticeable drop-off that is likely affecting him the most is how well he hits (or doesn’t hit) over the inside part of the plate. His highest wOBA on pitches on the inner third of the plate is .269 whereas in 2019 his wOBA was no lower than .350 in any quadrant on the inner part of the plate. It’s a surprising area for him to struggle.
This difference isn’t due to a change in pitch mix or pitch location either. He’s seeing slightly more fastballs (63.0% vs 60.1% in 2019) and slightly fewer breaking balls (28.3% vs 30.1%) and offspeed pitches (8.7% vs 9.9%). Pitchers still try to attack him low and away off the plate most often. So what he’s seeing is mostly consistent with what he’s used to.
He may even be a little more selective in pitches this season too. He’s swinging less overall (46.3% vs 51.4% last season) and a lot of that is swinging less at pitches in the zone (63.7% vs 73.1% in 2019).
He’s not whiffing on pitches much differently than we’re used to and he’s chasing pitches outside the zone slightly less than normal too. So plate discipline probably isn’t a likely reason for his struggles to open the season.
Rather, Nolan is quite simply getting under the ball more than he normally does and it’s leading to softer contact and subpar results.