Getting Patterson, an announcement made in the middle of Boston's 4-2 win 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.
"It will be great," manager Terry Francona said of Patterson's addition.
Patterson was initially set to join the team Sunday, an easy trip considering the A's are playing at home across the bay against the Pirates, but Francona said he would instead meet the Red Sox on Tuesday in Beantown.
Patterson appeared in 45 games for the A's, batting .204 with four home runs and nine RBIs, 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.