ISTANBUL — Doc Rivers is all for Rasheed Wallace returning to the NBA, and in fact never thought he should have retired.

Wallace, the skilled center who played in 79 games for the Celtics in the 2009-10 season, signed with the Knicks Wednesday after two years away from the game.

The last time we saw Wallace was after the Celtics’ Game 7 loss in the Finals to the Lakers at Staples Center; with his shoes off, he was knocking on the doors of the officials locker room, asking to speak with Dan Crawford. Wallace never got the chance, and he disappeared for a while before contemplating a comeback last season.

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He had been working out for a few weeks with the Knicks, who signed the 38-year-old as a backup center.

“He talked to me last year, so I knew he was thinking about it,” Rivers said. “Last year, he just couldn’t get in shape in time because of the lockout.

“I didn’t think he should have retired, honestly. I thought he had too much left to give. I just told him what I told him when he was here: ‘You’ve got to get in shape first.’

“I don’t know if [the skill] is still there or not. I hope not, since he’s with the Knicks.”

Remaining in shape was an issue during Wallace’s season in Boston. He reported to camp overweight and never quite got into premium condition. In that Game 7, Wallace fouled out, but before that asked out multiple times because he was winded.

“It’s all conditioning,” Rivers said. “To me, with older players, it’s always conditioning.”

Flop talkRivers said he is in favor of the league’s new penalties for flopping, which has become an increasing problem over the past few years. Players will be given a warning for the first flop, then fined $5,000 for the second violation and an increasing amount for further infractions. Officials will not have to rule on flopping during games.

“It’s a good rule,” said Rivers. “I think it’s just going to take some time to take shape.

“At least the refs don’t have to do it. I think that’s the best part. I think they have enough to do rather than sitting around worrying about who’s flopping. My goodness, at least we took that out of their hands.”

Reserve power

A team led by mostly reserves and camp invitees such as Kris Joseph, Dionte Christmas, and Fab Melo beat the Celtics’ front-line players, 14-11, in Thursday’s first scrimmage, a positive sign about the talent balance on the roster. The losing team was made up of the projected starting five — Kevin Garnett, Paul Pierce, Rajon Rondo, Courtney Lee, and Brandon Bass — sixth man Jason Terry, and the first forward off the bench, Jeff Green.

“It’s good because it’s only going to make us better,” said Pierce, referring to the strength of the reserves. “If you have a third team that can come out there and push the second unit and first unit, it’s only going to make your team better.

“Usually your third team, that’s always getting blown out, losing every game, but that’s not going to be the case here. I think these guys have a great makeup. They’re going to be good for us throughout the year.”

Wilcox day to day

Chris Wilcox missed his second consecutive practice with back spasms but did participate in other drills. He is listed as day to day . . . Micah Downs did not participate in practice because of a sore left shoulder. He is not expected to make the team, and the injury may damage his chances of getting a closer look . . . The players took to tossing a football around the court after practice. Green played receiver, running patterns against defensive back Lee, with Rondo at quarterback. Rondo was a high school quarterback in Louisville, Ky. . . . Pierce tweeted Wednesday night that he was looking for a good restaurant in Istanbul and told reporters that he found one . . . Rivers said two of the officials for tonight’s game against Fenerbahce Ulker will be from the NBA. The teams will play FIBA rules.