Boston Celtics big-man Al Horford has joined his All-Star teammate, Jaylen Brown, in COVID-19 protocols after testing positive for the virus.

Horford and Brown will miss the Celtics' preseason matchup with the Orlando Magic set for Wednesday.

Horford, 35, marks the third known COVID-19 case for the Celtics in recent weeks. Head coach Ime Udoka had a breakthrough case last month. Udoka completed his isolated quarantine and returned to the sideline prior to the preseason.

Brown's positive test came this past Friday. He was asymptomatic and placed in quarantine. The team said Horford is in isolation and “further updates will be provided as appropriate.”

Horford is set to start his second stint in Boston after being traded for Kemba Walker in June. He played for the Celtics from 2016-19 and earned one of his five career All-Star berths in 2017-18.

The Celtics begin their regular season on Wednesday, Oct. 20, with a visit to Madison Square Garden to face Walker and the New York Knicks. Both Brown and Horford should have time to complete their quarantines and produce multiple negative tests in order to be cleared to play in the season opener.

Horford has played in two preseason games thus far. He made 28 appearances for the Oklahoma City Thunder in 2020-21 before being shut down for the season despite being completely healthy.