The reliever said his late arrival is due to a positive COVID-19 test, though he added he's feeling better now.
"Take it serious," Jansen said about the virus. "Wear your mask at all times if (you) can. Trust me, it happens fast."
Jansen believes he caught the virus nearly three weeks ago from his 4-year-old son, who also tested positive, according to ESPN's Alden Gonzalez.
Even after a bout with COVID-19, Jansen doesn't intend to opt out of the 2020 campaign and thinks he'll be ready for the start of the regular season.
The three-time All-Star isn't the first high-profile closer to contract the virus, as Aroldis Chapman tested positive earlier in the week.