The 16-year-old rejected professional terms with the League Two side, opting to take up Chelsea's proposal instead.
Tisdale told supporters at a Q&A session: "It's a very difficult process – it's uncharted territory for us, a player at that level who's gone to one of those clubs.
"I have to be careful what I say because it still may go to a legal situation.
"Ethan had the opportunity to leave at that point, we didn't sell him – he didn't sign his scholarship forms, he wanted to leave, and he had the pick of so many clubs, as you can imagine.
"He chose Chelsea so then you've got a situation where we either agree on a fee with Chelsea, which is an ongoing process, but now they've got him they're probably not going to offer the sort of figures we want.
"So it will either be settled shortly with an agreement between the two clubs, or it will go to tribunal, which will be set in the future, maybe not straight away, and then we'll have a tribunal situation where us and Chelsea will sit down and there'll be a fee that's given and Chelsea will be asked to pay that money.
"We really don't know – we know how good Ethan is and how good he will be, and we have to hope that we can either get an offer from Chelsea that we see is appropriate or we get the right figure from a tribunal.
"We're open minded either way, it just means that, at the moment, we cannot spend money until we know what we've got.
"We know we're going to get something, we'll be hopeful for a high amount, but we don't know yet."