Jairo Diaz had a rough 2020 season and he has had a rough spring too. In 2020, he went 1-2 with a 7.65 ERA in 24 games. He also had a FIP of 6.64, a WHIP of 2.25, and saw his walk rate more than double from 3.0 to 6.3 BB/9 and his strikeout rate dropped from 9.8 to 7.7 K/9.
In spring training thus far, he has appeared in five games and allowed nine hits, six runs (all earned), three walks, and he has struck out three. Not necessarily the stats you want to prove yourself.
He still will likely make the bullpen for Opening Day but if he keeps pitching poorly throughout the rest of spring training, the Rockies may decide to ride with younger pitchers that have shown more recently.