Gerald Wallace is the most senior member of the Celtics with 12 years of NBA experience on his resume. So when the veteran swingman speaks his words carry weight in the locker room.

And Wallace was not at all happy with how his team played in a 104-89 preseason loss to the Minnesota Timberwolves at the Bell Centre Sunday a game that was part of the NBA Canada series.

It was the Celtics’ sixth loss in seven preseason games. They have one exhibition game left Wednesday against Brooklyn at TD Garden and their regular season opens Oct. 30 against the Raptors in Toronto.

“We’re not playing with effort” Wallace said as he sat in his locker one of the last in the room before the Celtics boarded their charter flight back to Boston. “Guys are out there being selfish. The opponent is giving it their all regardless of how the night is going.

“[Sunday] we ran into another team that just wanted the game more than we did and they came out and played like it. We had some good spurts but when it came down to it they were determined to win the game and we weren’t.”

Can effort be taught — or instilled?

“Listen I mean veteran rookie whatever you want to call it I was always taught you can’t teach effort” said Wallace who scored a game-high 16 points on 6-of-12 shooting in 26 minutes while also tallying 3 assists 2 steals and 2 rebounds.

“You can’t teach somebody how to give effort. That’s easy. They’ve got that or they don’t. The main thing about it is either you want to win or you don’t want to win. Effort isn’t something you can learn or teach or give somebody or encourage somebody.

“When we step out on the basketball court we get paid to do this. We’re professionals. So our main thing is you should want to go out every night and win. It shouldn’t be a question of the effort. You’re going to miss shots you’re going to turn the ball over things aren’t going to go your way but it shouldn’t be because you’re not playing hard or you’re not giving your all.”

The Celtics are learning a new system under rookie coach Brad Stevens and he and the players recently have noted that despite the losses it is still the preseason a period for adjustment and growth.

But according to Wallace they no longer can use the word “preseason” as an excuse.

“You know what the first couple games you can say that” Wallace said. “Right now you can’t say that. We’ve got one more preseason game before this thing really starts up. If guys really think they can just wait until the Toronto game to be able to turn the switch on it’s not that easy in this league. You’ve got to start building yourself off and get some momentum going into it. We’re not doing it right now.”

Wallace added “We’re playing teams and we’re concentrating on it being ‘preseason.’ We’re trying to make an excuse. We’re playing lackadaisical. These teams are coming out wanting to win and get better. Right now we’re not doing that. We’re just going through the motions it seems like without effort. We can’t afford to do that.”

Jeff Green who had 12 points off the bench agreed and said the effort wasn’t there.

“We need to focus a little bit more now treat this next game these next couple practices as the regular season because I think it’s too late to blame the whole ‘working out kinks’ statement that we’ve been making for these past couple games” Green said. “At this point we know what to do. We know our offense. We’ve just got to go out there and play. We can’t come into this thinking ‘It’s preseason we can [play half-heartedly].

“We’ve got to be ready to play. Teams are getting geared up for the regular season. We can’t allow ourselves to feed into it just being preseason.”