If the Yankees win one of the first two games, then they are guaranteed a tie for the AL East title and will probably play the Sunday game without doing anything dramatic in terms of their pitching. If they go into the Sunday game needing a win to tie Boston for the AL East title (especially if Cleveland is ahead or tied for the wild card lead), then they'll pull out all the stops and use a dozen different pitchers on Sunday if necessary -- which means that they'd be very thin for a Monday game if they win on Sunday.
If that kind of scenario unfolds, then the Yankees could end up in a position kind of similar to what they faced in the playoffs last year, when they were up 3-0 against Boston but actually faced a daunting task for the remainder of the series -- because a rainout early in the series ensured that the Yankees' lack of depth in the starting rotation would become a real problem when they had to play something like five nights in a row.
Pitching for both teams is shaky.