Solanke has been missing since August with an ankle injury. He has since been replaced by Mathys Tel in Spurs' Champions League squad.
After their win against Slavia Prague, Frank said: "Because, as I understand it, it's something with 60 days. An injured player needs to be injured for 60 days, relatively complicated, so it's not like I could have done it three games ago.
"Plus, it's also depending on how long will Dom be out and so on. So, that's why there was an opening now, we could do it and we can change it again, so that was good.
"It was just an opportunity to get Tel in, which is nice to have that opportunity. We can change back if we want to do that."
Asked if Solanke can make the Eintracht Frankfurt game, Frank added: "I think I've said before that as soon as he's ready and available, then I will say when he's ready."