If the series goes to a fourth and fifth game, Lester could be available to pitch on short rest and Beckett would be on his regular rest for the finale.
Red Sox manager Terry Francona made the announcement after Sunday's 12-7 victory over the Cleveland Indians in the regular-season finale.
Francona, who said earlier in the day that Tim Wakefield would not be on the ALDS roster, said that other roster decisions still had to be made.
The Red Sox don't know yet if they'll open against the Angels on Wednesday or Thursday.
Information from The Associated Press was used in this report.