Not exactly an impressive resume when you take a closer look. The Cavs game was over after a 32-18 1st quarter, in a game where Luke Walton played 18 minutes. Granted Mills scored efficiently, but it wasn't exactly against top level players or subs.
Then there is the Suns game in which Mills went 9-23 for 27 points while both Channing Frye and Grant Hill were injured and didn't play, and Nash played a total of 17 minutes. To put things in perspective Josh Childress, who the money conscious Suns felt was worth amnestying this off-season, played 21 minutes that same game. In fact with the exception of Ronnie Price, no one on the Suns played fewer minutes than Nash and yet they only lost by 4.
Then there was the final game against GSW. Again keep in mind this was a GSW team that had to lose in order to have a chance at keeping their lottery pick. So in order to do that they started 5 rookies, played a total of 7 players with Chris Wright and Jeremy Tyler logging 46 and 44 minutes respectively.
That's not to say the Spurs were at full strength, since the Big 3 didn't play in any of those games, and Splitter and Green played only limited minutes in the GSW win. However none of those games are particularly good indicators of how he will workout as a full-time backup PG for the Spurs.