Although Saverio's post wasn't specific about whether the sideboard cards were unique to the sideboard, or additional copies of cards also found in the main deck, I didn't want to risk a complete answer here until the KP scenario had been concluded. (I encourage everyone to follow
this link to that KP scenario, for that answer.)
If, as noted in the KP scenario, the sideboard cards duplicate one or more main deck cards, the downgrade to Warning does not apply, and this is simply a Game Loss - thus, mulligan is no longer relevant.
However, if we do apply the downgrade, we now have a starting hand of 5; the two sideboard cards (and any others in the deck) cease to exist. This has two ramifications:
1) if you choose to mulligan, you will mulligan to one less than your current hand size - in this example, to 4.
2) if you do not choose to mulligan, you can still perform the “scry” (CR 103.4 doesn't actually call it that; in a related post, Florian referred to it as the “Vancouver Scry”, since it originated at Pro Tour Vancouver, August 2015), because your starting hand is less than the 7 you originally had.
d:^D
P.S. - another thread was started, asking very much the same questions; I'm
linking to it here, just for completeness. I also ask - and this is an age-old standard protocol for these forums, and the Judge List before them - that you do a bit of research
before you post, in case your question has already been posted.