An MRI revealed that key striker Danny Koevermans, injured in a collision Saturday with New England defender A.J. Soares, has a torn anterior cruciate ligament in his left knee.
The 33-year-old Dutchman will be out the rest of the season and could miss a chunk of the next one given the normal lengthy recovery time for ACL surgery.
Toronto FC said there was no date for the surgery yet. And in a brief statement Monday, the club didn't provide a timetable for Koevermans' return.
The team had the day off Monday and Mariner declined an interview request.
The Koevermans injury adds another hole for Mariner, in his role as director of soccer operations, and Earl Cochrane, director of team and player operations, to fill.
While Toronto (4-11-4) lacks depth on all fronts, their focus had been on strengthening the backline. Now the attack needs help.
The injury leaves Jamaican international Ryan Johnson as the team's main striker. With the recent release of Nick Soolsma and sending out on loan of Joao Plata, Mariner has few attacking options other than newly acquired Andrew Wiedeman and rookie Luis Silva, although Reggie Lambe and Eric Avila can both push up from midfield.
The situation is complicated somewhat in that Mariner has shifted the team to a 4-4-2 formation, using two strikers. Under former coach Aron Winter, TFC played a 4-3-3 which allowed greater use of attacking midfielders.
Koevermans was hurt as he chased a through ball on the right side of the penalty area in the 37th minute. As he pulled up to cross the ball, he collided with Soares who was rushing back into the box to catch him.
Soares appeared to accidentally step on Koevermans' left foot as they made contact, seemingly jamming it into the artificial turf.
The six-foot-three Dutchman, who was cut from taking a cleat to the forehead in the collision, was stretchered off and left Gillette Stadium on crutches.
"I fear the worst for Danny," Mariner said after the game. "I've got my fingers crossed, but I spoke to the doctor from New England, who I have the highest regard for, and it doesn't sound too good."
Koevermans is tied for fifth in league goal-scoring with nine this season and his rich vein of form in recent weeks has helped the team's turnaround under Mariner.
His goal total is all the more impressive given that he missed a chunk of the first half of the season through injury and was slow to find the goal upon his return.
Then he hit a purple patch, scoring goals with his head, and almost every other part of his body, including a creative flick of the leg.
That last goal, in a 1-1 tie with FC Dallas on July 4, was his sixth goal in five games.
The Dutchman has 17 goals in 26 MLS games since joining the team with fellow designated player Torsten Frings midway through last season.
He has 19 goals in all competitions for TFC.
Toronto hosts the Colorado Rapids (7-11-1) at BMO Field on Wednesday.