Out since April 13 after suffering a subluxation (separation) of his right shoulder vs. the Rays, Ellsbury could join fellow outfielder Carl Crawford in the GCL.
"It's great," said manager Bobby Valentine. "I'm cautious, but I think it's great that we've gotten him to the point where he's back on the field. That means you start looking forward to the time he's on the field here. But he's not on the field yet."
Ellsbury took batting practice at Fenway Park prior to Tuesday's game against the Blue Jays. He also shagged fly balls and played catch.
"[Hitting coach] Dave Magadan really likes the way he's looking," Valentine said. "I saw him yesterday. He looked strong."
The plan is for Ellsbury to start his rehab assignment as a designated hitter on Friday, and then later transition to playing center field.
Carl Crawford began his rehab assignment with the GCL Red Sox on Saturday, going 0-for-3 with two walks and a run scored as the DH. The GCL Red Sox were rained out for the second straight day on Tuesday, limiting Crawford to batting practice.
On Tuesday, the Red Sox contemplated moving Crawford to another Minor League affiliate to get him on the field and avoid another possible rainout. He's recovering from a UCL sprain in his left elbow and also had offseason surgery on his left wrist.
"We're trying to figure out if it's better to move him to play," Valentine said. "They say the storm is coming up the Northeast. I don't know. I think Carl was ready to play in the field when the rain hit."
As for injured outfielder Ryan Sweeney, on the disabled list since June 17 with left big toe inflammation, he's progressing and is no longer wearing a walking boot on his left foot.
"He had special orthotics made for his left inner sole," Valentine said. "He has some issues with the bottom part of his leg that are being manipulated and dealt with to try to alleviate the problem going forward."