The former Wigan player sustained the injury just minutes after his introduction as a substitute in Sunday’s game at Wakefield and scans have confirmed the worst fears of coach Tim Sheens
The club say the 27-year-old Greenwood, who is out of contract at the end of the season, will undergo surgery in the coming weeks.
A statement said: “The club can confirm that James Greenwood has undergone scans following an injury sustained in last weekend’s game against Wakefield Trinity.
“The second row was helped from the field and did not return to action during the round five tie.
“Scans have confirmed that he has ruptured his anterior cruciate ligament and will have surgery in the coming weeks. He is not expected to feature again in 2019.”
The Robins have also announced that forward Ryan Lannon, signed from Salford in the winter, will be out for 10 weeks following ankle surgery.