The striker limped off in the goalless draw at Portsmouth and it was initially hoped that he would be back in action by April, but a scan has revealed the 26-year-old suffered a severe tear.
Boro coach Mark Proctor told the Evening Gazette: "Hamstring injuries are graded, and unfortunately Leroy has suffered the highest grade tear.
"You can't say for definite but it's possible he won't play again this season. Obviously it's a blow for Leroy. He had been going very well and scoring goals.
"But injuries are part and parcel of the game and, when one player gets injured, it gives an opportunity to somebody else to step in.
"We have a few players who can do a job up front like (Marvin) Emnes, (Merouane) Zemmama and (Tarmo) Kink, and there will be an opportunity for one of them against Watford on Saturday."
Source: PA
Source: PA