Firstly, acknowledge that it is very disappointing to lose a game. However, the next immediate step is to draw attention to the results of playing such a game. We lose some, some we win but the joy is in the company of playing.
This has to be done during neutral moments, elaborately, and anecdotally as a story. Before the game starts, its good to draw the attention to such probabilities, by showing the toss of a coin, a few times, as an example, to show that not all the time a single face appears. This, if done once, will help the child realize that winning is by a good playing effort, and by chance too, which could happen to her/him also. If the child is not ready to accept it, you can ask to pause for such games, that she/he is unable to accept losing. But stressing on the efforts in the game, the child will know that the chances are equal for all players.