I think he'll be able to defend a lot of 2s, particularly when he gets up to speed - no matter how much he's been working out, you can't duplicate NBA situations. But I think what you're talking about is the beginning of the second quarter. It just so happens that the Pistons made a smart substitution and Doc didn't make a countermove. Prince came in for Hamilton at 9:40 in the second quarter, switching Herrman, a 6'9" 225 pounder who usually plays the 3 or 4, to the 2. Marbury will be able to work up the quickness to stay with guys like Hamilton just can't cover someone that much bigger than him.
What's astounding is that Doc stuck with the small backcourt of Marbury and House for over three minutes after that substitution (ultimately, Ray for Marbury at 6:20), during which time Herrman hit two easy shots against the smaller Marbury who just couldn't contest him, and made it to the line on another occasion. Why did he wait 3:20 to make a change? If you want to rest Ray, put Walker or Giddens, or even Davis (switching Moore onto Herrman and putting Davis/Powe on Kwame and McDyess/Maxiell) in for House to get some height on Herrman. It's not like he was running any plays to get House open for 3s against a taller defender who couldn't keep with him if he kept running off screens.
Fortunately, Doc learned a bit, since in the 4th, with basically the same lineups (Marbury, House, Pierce, Powe and Davis v. Bynum, Herrman, Prince, Maxiell and McDyess), Boston started running plays for House and he responded with 2 3s in the first 2 minutes, turning a 7 point deficit into a 2 point lead. You also have to give credit for the fact that during that run, Marbury and the rest of the team played good enough defense to get the Celtics a nice run - with that lineup, the Pistons settled for 4 jumpers, making just 1, had a shot clock violation and one driving layup when Bynum burned past House - forcing Detroit to make the switch (which they intelligently did), bringing in Stuckey, Hamilton and Wallace for Bynum, Herrman and Maxiell. I just wish Doc had made a similar switch earlier in the game when it was necessary, or was smart enough to run House around screens to get open against bigger, slower defenders.
Bottom line, Doc had as much or more of a bad game than Steph.