Getting Patterson, an announcement made in the middle of Boston's game at San Francisco on Saturday, gives the Red Sox some added depth after second baseman Dustin Pedroia was placed on the 15-day disabled list with a broken left foot.
Patterson was set to join the team Sunday in San Francisco. The A's are playing at home across the Bay against the Pirates. Patterson has appeared in 45 games for the A's, batting .204 with four home runs and nine RBI, two triples and five doubles. He has six stolen bases.
Williamson was 4-3 with a 3.72 ERA in 14 starts for Single-A Salem.
Boston's 40-man roster is now at 40.