The Dutch striker, who has been out for more than 10 months due to knee surgery, is in doubt for his comeback game Saturday against the visiting Philadelphia Union.
"He twinged his other knee, so he's in doubt," manager Ryan Nelsen said Wednesday after Koevermans missed practice. "It's a really minor thing."
Koevermans, 34, has not played since tearing up his knee last July in New England. Toronto has gone 2-17-8 in MLS play in his absence.
The big striker, who has 17 goals in 26 games for TFC, has been raring to go for weeks and has looked sleek in practice.
Backup goalkeeper Stefan Frei missed practice again after being injured in practice last week. At the time, he said he had been cleated but the problem is now said to be a hamstring.
Frei was slated to have a scan later Wednesday. Nelsen is hopeful the injury is minor and that the goalie should be available next week.
The news is not so good for fullback Richard Eckersley, who has been out since April 6 after injuring his hamstring celebrating a goal by teammate Darel Russell.
The English defender, who has missed nine games already, re-tore the hamstring.
"Eckersley's a long way away," said Nelsen.
Logan Emory is sidelined by a bone bruise on his foot while fellow defender Danny Califf, the victim of a virus, is expected back in training Thursday.
Striker Justin Braun, coming back from an ankle knock, trained by himself Wednesday. Nelsen called him day-to-day.
The good news for Nelsen is that Russell is back training, giving him a choice of three fullbacks including Ryan Richter and Ashtone Morgan.
Nelsen has shifted captain Darren O'Dea from centre back to left back the last two games, playing Steven Caldwell and Doneil Henry in the middle with Richter on the right.
O'Dea was on the bench Wednesday as Ireland played England to a 1-1 draw in a high-profile friendly at Wembley. He is due to fly back to Toronto for TFC's game Saturday before returning to Dublin for games next week against Georgia (a friendly) and the Faroe Islands (a World Cup qualifier).
It was O'Dea's decision to return for the Toronto game.
Nelsen seemed torn on what to do Saturday with O'Dea, given his captain's commitment in returning to Toronto between games and the air miles that come with it.
With O'Dea not playing Wednesday, chances are he will see action on the weekend in Toronto, although Nelsen talked up Morgan's recent form.
Toronto (1-7-4) is winless in 10 league games and has lost its last five. Philadelphia (5-5-3) is coming off a 5-3 loss in Montreal last week although it defeated the Ocean City Nor'Easters 2-1 in U.S. Open Cup play Tuesday.