According to Theo Epstein, Josh Beckett threw a side session on Monday at Fenway Park before the team left for Denver.
The righthander has been on the disabled list with a bad back since May 18. He has appeared in only game since May 7.
Beckett is expected to need another bullpen session or two before he embarks on a minor-league rehabilitation assignment.
The earliest he could return is roughly the first week of July. But it is likely to be later than that as the Sox will want to make sure Beckett is OK and not in danger of altering his mechanics.