The big problem with the Spurs' back up PG spot is that, on paper, every option should work just fine. Neal is the shooter that you can slot at PG but play off the ball while Manu runs things. With the Spurs going away from an almost exclusively pick and roll oriented offense Neal can also benefit from assisted looks off screens. De Colo's court vision and good handles make him a good playmaker. His chemistry with Tiago Splitter in the pick and roll gives the Spurs a good fall back option if the ball is forced off Manu's hands. Patty Mills' foot speed and energy allow him to put pressure on ball handlers and his shooting needs to be respected by opponents.
In practice, however, we are seeing that there is a downside to each of these players. Neal is a drain on defense and when his shot isn't falling brings little to the table. De Colo is, at this point, basically useless off the ball. Mills' defense is more energetic than actually good and his shot selection is often...problematic.
Pop has leaned towards Neal and it's easy to see why. Assuming Manu Ginobili is healthy in the postseason, he and Tony Parker with take most of the playmaking duties. One of them will likely be on the court at all times and taking the ball out of their hands would be a mistake. Neal is also a very turnover-averse player and, while he might struggle to get the ball up court against pressure, he can serve as a secondary creator from time to time.
In reality, who plays back up PG is the least of the Spurs' concerns on offense. As long as there is movement off the ball and the floor is spaced well the Spurs will get quality looks. If things break down, Ginobili or Parker can create something. In that sense, the bigs are more important than the role playing guards. So that leaves defense.
The three back up PGs have the worst on court defensive rating of all current Spurs. When Mills, De Colo or Neal are on the court, the Spurs allow over 100 points per 100 possessions. Mills has the best net rating of the three with a 6.6 and De Colo the worst at measly 0.4. Neal ranks in the middle at 4.8. The problem is, the Spurs seem to be much better when Neal is off the court. Their defensive rating gets to a fantastic 96.3 and they outscore their opponents by 12 points per 100 possessions. De Colo is similarly bad but Mills does a bit better, with the team still doing better when he is off the court but not as well as it does without the other two playing.
Since these guys are replacing Parker, the player who by the numbers has the biggest impact on how the Spurs do on both ends of the court, it's not shocking to see their on/off numbers suffer but it definitely seems they are some of the least effective players on the roster. Their individual numbers aren't great either.
They are all shooting under 40% from behind the arc, with Mills having the best mark on fewer attempts than Neal, who is having his worst year shooting the ball. De Colo and Neal have solid assist to turnover ratios but get them in opposite ways: Neal doesn't assist a lot but doesn't turn it over, either and De Colo turns it over way too much but manages to offset that by generating a lot of buckets for his teammates. De Colo is the best rebounder off the bunch but he's nothing special and only Mills has good overall shooting numbers. They don't get to the line and don't get steals.
Considering those parameters, you could say back up PG is a weakness even if the Spurs have four roster spots (counting Joseph) devoted to it. You could make the argument that Neal is in fact not a natural PG but he has been playing that position for most of his tenure with the Spurs. Once again, back up PG is not that important a role since Ginobili is basically a scoring point guard at this point but coming into the playoffs even small weaknesses can be exploited.
It seems Gary Neal will be the back up PG when the post season rolls around and after watching the uneven play off both the Aussie speedster and Nando De Colo it's hard to blame Pop for his decision. While Mills might edge out Neal according to the numbers and there's a case to be made for him to supplant him, The Nail has proven his worth in high pressure situations and this season the Spurs seemed to be preparing for the playoffs from the get-go. It's probably why Stephen Jackson and Boris Diaw have maintained their spots in the rotation despite below average production: Pop simply trusts them more to produce when it counts.
Having a natural shooter with a shaky handle backing up Parker might not be ideal, especially when facing teams that can put pressure on Neal but the other possible candidates for that role are not without flaws and haven't exactly made a strong case to take his place so far, at least not in Pop's eyes. With only 24 games left, I doubt anything they do could change his mind but if there is in fact a change in the back up point guard role it won't likely make a huge difference on overall team play.
Stats via NBA.com/Stats