If I was in Dane Scarlett's shoes I'd be asking my agent to find a new club in January because if he ain't getting a look in now, I doubt it's going to happen for him at Spurs. Mathys Tel keeping him out of the team. After this season he'll be on his final one, so I suppose he could just wait and leave on a free transfer with more options but presently he's not doing anything and an important part of his career is slipping away.
I suppose if Tel hadn't been paid for and his relatively high salary, the club probably don't feel right paying that sum, and then giving Scarlett preference. So effectively Tel had ended Scarlett's hopes, unless he gets injured. I don't think Frank wants to piss off the higher ups at the club by playing Scarlett instead and having a relatively high wage player on the bench.