Have you had any temps close to freezing lately?

Why would you think it was overwatered? Have you watered it continually since planting? How long since you planted it? What's the weather been like? Is the tree in full sun? What soil do you have?

You really don't give us much to go on showing one limp leaf with that question.....

American Sycamore can tolerate a lot of water, they are well adapted to boggy sites. I doubt you are overwatering it. Could be a touch of Sycamore anthracnose, it's hard to tell for sure but it's not unusual this time of year if the weather has been cool and somewhat rainy. It doesn't look like anything I'd worry about.

I was going to say as Greg -- really hard to overwater Amer sycamore. It's new growth is very vulnerable in spring to both freezing temps and sycamore anthracnose.

Since you’ve posted in Container Gardening, does that mean you are trying to grow this tree in a pot?

got to see the whole plant including the pot ..

location.. big city name ... z6 is about half the US ... just include it in every post.. or change your name line to include such .. like ours.. i recognize your name .... but i cant commit everyones location to memory ...

one leaf doesnt mean much.. and it sure seems i can see a lot of perfect leaves in your pic ...

ken