Did they come in the same folder? How many textures are we talking about? When you import the FBX into Unity are there any Materials generated? If so what are their names. Do they reference the names of the textures?
If the texture number is less than 10 I’d say roll up your sleeves and match the textures to the right materials by hand. If that’s too much trouble put the textures and the FBX file in the same folder and re-import the file to see if the correct connections between the textures and their corresponding materials are made. Or if the file was given to you in some other folder hierarchy try replicating that structure in Unity and re-importing the FBX file. First make sure to delete all materials that were previously generated by the previous import attempt.