The Croatian playmaker wanted a big-money move to London rivals Chelsea in the summer but Spurs made it clear he was not for sale and knocked back any offers that came their way.
Even though the Blues have since said they are unlikely to again pursue Modric, Defoe feels that all the transfer speculation surrounding his mate will come out again as winter gets closer.
“There’s always speculation, it’s part and parcel of the game, and it will start again in January," Defoe told TalkSPORT.
"We just wanted the window to close, and he’s here now and that’s that. He’s just got to be professional. You can’t go into games sulking because you’ll get found out and let the team down.
"But Luka’s a good professional and, if you didn’t know there was speculation about him going and you saw him about the place, you’d never know.
"He’s been good. In pre-season, maybe other players would have said, ‘I’ve got an injury and I don’t want to play’, but he didn‘t do that and I don’t think there’s many players who would have done that.
"Chelsea’s a massive club and there was an opportunity for him but it didn’t happen and now he’s got to get on with playing football because he’s a big player for us."