Diaz (shoulder) threw a bullpen session Tuesday, Mike Puma of the New York Post reports.
Diaz threw 21 pitches during his mound session, and manager Carlos Mendoza said afterward that there is a good chance the 30-year-old reliever begins a rehab assignment in the minors Thursday. It's unclear how many minor-league reps the Mets will give their star closer, but the fact Diaz is already able to begin an assignment likely signals that he will only require a minimum-length stint on the IL.