When to repair engineered floor truss damaged by cut?
Well, it doesn't matter how thick it is, but it does matter how deep it is. If you've only cut one or two trusses, and it's no more than 1/2" deep, I don't think you have a problem. It's extremely unlikely that it'll cause failure of the floor system in any substantial way. 

However, since you have fairly easy access, I'd attach a repair member to the underside of that board, fastening it with construction adhesive and 3" gold screws (through the subfloor where necessary). Make it as long as can comfortably be fit in the space available.