Coach Derek Fisher missed practice Monday for what they Knicks termed personal reasons. In a more alarming development, Arron Afflalo missed practice for hamstring reasons — the third straight day he has sat out.

It looks as if Afflalo will miss the preseason opener against the Brazilian club Bauru on Wednesday at the Garden, and may be held out Friday in Washington, where the Knicks face their first NBA opponent.

Afflalo, the veteran shooting guard, was one of the Knicks’ key free-agent signees. He turns 30 years old next week and is coming off one of his worst seasons, split between Denver and Portland.

“It’s a day-to-day kind of thing with a hamstring, you have to be very cautious with it,’’ said associate head coach Kurt Rambis, filling in for Fisher. “We want to make sure he’s feeling really well before we throw him out for practice. We’re going to bring him along slow. We want to be very cautious with him.’’

The starting shooting guard slot is hardly a given for Afflalo despite signing a two-year, $16 million contract. He never has played the triangle, so his missed time doesn’t bode well. He could be the sixth man or play small forward in a small starting lineup with Carmelo Anthony at power forward. Langston Galloway could be in position to start at shooting guard Wednesday, with Jose Calderon at point guard.

“He knows how to play, [but] it’s going to be a learning curve when he get back there because he’ll play a couple of different positions,’’ Rambis said. “He understands our offense and played against it. He’s a quick learner and is asking questions when he’s not even out there.’’

Afflalo had said Saturday he expected to return to practice when the club resumed sessions Monday.

“The training staff will do a great job of bringing him along slowly and not just throw him out at practice,’’ Rambis said. “He’ll have to pass strength tests, agility tests before he gets to practice.”

The Knicks have reported no medical issues from their stay at West Point, which released a statement regarding traces of Legionnaires’ Disease found in some of their cooling towers and collection basins used for heating, ventilation and air conditioning. In a long statement, Col. Landy Dunham said, “The danger to the public is minimal.’’

Rambis said the players have caught onto the triangle more rapidly than they did during last year’s camp.

“These guys were all handpicked for attributes they bring,” he said. “One of them is they all have high basketball IQs.’’