Except according to Solskjaer himself Lingard was not fit to play a full game, yet capable of playing some minutes. That was Solskjaer's opinion and even just based on that opinion alone, bringing Lingard on so early was a big risk.
If your logic is correct (that you're either fit to play or your not) then what Solskjaer said before the game also made no sense. Either he believed what he said (in which case he was at odds with your logic) or he didn't believe it (in which case he didn't actually think Lingard was fit to play at all but had him on the bench anyway).