The 2-1 loss to Pompey on Saturday, coming after the previous weekend's 2-0 loss to Oxford, saw the Brewers beaten in successive games at the Pirelli Stadium for the first time since April 2012.
Pickford played after rejoining the club on loan from Sunderland and the young goalkeeper produced some fine saves to keep the visitors at bay.
Diamond, meanwhile, started his first game of the season after recovering from a foot injury and the defender marked a decent display with Burton's consolation goal deep into added time.
"Those two were excellent, but after that I'm struggling to name too many who were very good on the day," said Rowett.
"I thought Zander acquitted himself well and Jordan made some excellent saves and couldn't do an awful lot on the goals - maybe he needed a little more help from one or two defenders.
"Again, that's twice they reacted quicker than us and that's not really acceptable."