The Frenchman is interesting Ligue 1 Paris FC, which wish to take the striker on-loan for the remainder of the season.
But Frank said on Friday: "It's fair to say we have not many players.
"The players we have we need to keep. In the Dortmund game with not many available players, it's difficult to change the game when you need it or keep the freshness up."
Tel has been omitted from Tottenham's Champions League squad for the second-half of the season in preference for fit-again striker Dominic Solanke.
"It's very tough for Mathys," said Frank. "My challenge was that it could only be him or Dom. I couldn't sub another player out. If so, I would have done it.
"It's not nice but it's a football decision. It's not the same as saying I don't like Mathys at all. I chose Dom because of his experience. Luckily, he did very well.
"Unfortunately, it impacts on others. Mathys in this case, and we need to get around him. He was disappointed, that's natural. But he's been very professional. He's a strong character.
"He has done well for us in many ways and started four of the last five, that's a clear signal I trust him and believe he can do something good."