A Red Sox source said no decision has been made on the corresponding roster move. There is an open roster spot after the demotion of Lars Anderson last night. But the Red Sox also plan to add RHP Aaron Cook to the roster.
The move for Cook may not come until Friday. He had the right to opt out of his minor league deal on Tuesday, but the Sox had 48 hours to make a roster move for him.
Youkilis has missed the last three games with a back injury.
The 23-year-old Middlebrooks is hitting .333/.380/.677 with Pawtucket this season. He has nine home runs and 27 RBIs in 24 games.