Carell's Office return unlikely
The comedy series is coming to an end this year after an eight-year run, and there had been rumours that original star Steve, who quit the role of bumbling office manager Michael Scott in 2011, could pop up in the final show.
Speaking at the network's Television Critics Association winter press tour, Greenblatt said: "I'm hopeful but I don't think he will be back.
"He left in the way that he wanted that character to leave, and I think he and Greg (executive producer Greg Daniels) have talked about it and I don't think he'll be back.
"There's maybe a little Hail Mary pass on a cameo, but I think the decision is that it'll go out without him rather than compromising his exit."
The award-winning series currently stars John Krasinksi and Jenna Fischer.