It looks like wind damage.  The leaves are large and soft and when exposed to strong winds, they rub and abrade each other.  Cucumber leaves are sandpaper-like to the touch so it is not unusual for plants raised in greenhouses or indoors to have big, soft leaves.  It appears to be only a few leaves that are damaged.

It does not to appear to be to disease because there are no defined spots or rings or colored margins on the spots. Disease would continue to spread.  It's a little too early, also.