Arron Afflalo has told friends he has longed to play in Madison Square Garden’s electric environment. Now, he finally gets the chance as the Afflalo Era dawns Friday against the Celtics.

So ecstatic to sign with the Knicks in July, Afflalo has been an invisible figure in preseason, suffering a left hamstring strain on the third day of training camp at West Point and not seen again. Until Friday.

Afflalo went through all of practice and his first scrimmage Thursday since the Sept. 30 injury. After missing the first three exhibition games, the Knicks’ second-most important summer signing will be listed as probable for his Knicks debut.

Afflalo, who inked a two-year, $16 million free-agent contract this offseason, likely will start at shooting guard and play alongside his former Denver wing man Carmelo Anthony — as long as his hamstring isn’t too sore from Thursday’s work.

“It felt good,’’ Afflalo said of the scrimmage. “I had an extended vacation. The training staff has been great with me in terms of rehab. Now it’s getting acclimated with my teammates.’’

Afflalo said hamstring injuries such as his normally take two to three weeks to heal, and claimed he suffered no setbacks.

“It’s a positive anxiety,’’ Afflalo said of his debut. “I got two days of training camp in, so it seems like it’s been awhile. I’m familiar [with the offense]. I’ve been standing here watching every day, so I’m excited.

“My wind is great because I’ve been doing sprints, conditioning, weight training. It’s more or less getting a continuity with my teammates. As you know, the triangle offense takes some time to get on the same page. I want to play at a good game speed and not feel so frantic out there.’’

Coach Derek Fisher said he was happy with the way Afflalo ran during the scrimmage. Afflalo is projected as the starting shooting guard and will play some small forward in the team’s small-ball unit. Fisher thinks starting him may be the better way to get this rolling so he’ll still be limber after warm-ups.

“He moved well today,’’ Fisher said. “He’s moving in the right direction. We’ll see how he responds.’’

Fisher was gung-ho over signing Afflalo because of his defensive grit, something which was sorely lacking during last season’s 17-65 nightmare. Some league executives felt the Knicks got good contract value, even if Afflalo is 30 years old and coming off one of his worst seasons split between Denver and Portland.

“His character,’’ Fisher said in listing reasons why Afflalo became an offseason priority. “He’s a team guy and he competes on both ends of the floor — sounds strange to say a guy competes on offense and defense. There’s a level of physicality and aggression Arron plays with that’s good for us. He obviously hasn’t played basketball in several months. So we won’t see the best of him for a while.”

Team president Phil Jackson had made Wesley Matthews his top free-agent priority until Matthews tore his Achilles last spring and still wanted to break the bank. Danny Green also was of interest but wasn’t willing to give his hometown team a discount. DeMarre Carroll priced himself out of the Knicks’ range.

The Knicks, meanwhile, had known Afflalo wanted to play in New York, requesting a trade there at February’s deadline before the Nuggets shipped him to Portland.

“He has the capability of playing aggressive defense on big guards and small forwards, plus he’s a shot-maker,” Jackson told ESPN. “In addition to his 3-point range, Arron can be very effective scoring in the low post and at the pinch post.”

Fisher said he’d probably play Afflalo limited minutes Friday, so he also can play him Saturday in Charlotte. After Charlotte, the Knicks will be have one preseason game remaining.

The sight of Afflalo and Anthony playing off each other again will be Friday’s highlight. On a roster lacking go-to scorers, Afflalo is the best bet to be Anthony’s secondary option. To date, Anthony has looked supreme in his comeback from knee surgery.

“His jumper looks sweet,’’ Afflalo said. “I know he’s excited, having not played since last February. I thought in all of the games, he had his spurts. He looked at his best.’’