Graham Dorrans scored the game's only goal from the penalty spot in the 13th minute at Bonney Field on Monday.
Other than a bright start, which included the goal, Alan Irvine's men battled against a team in the third tier in the United States.
Making matters worse, midfielder Craig Gardner was sent off with eight minutes remaining.
Among the players missing for the Premier League side were Joleon Lescott and Gareth McAuley (calves), James Morrison (knee), Chris Brunt (groin), Craig Dawson (thigh) and goalkeeper Ben Foster.
West Brom, in the USA on the back of consecutive friendly wins, unsettled the hosts early.
Sacramento forward Max Alvarez tested Boaz Myhill from range as the hosts also looked threatening in the attacking third.
The visitors took the lead in the 13th minute after Montenegrin defender Nemanja Vukovic was penalised for pulling Victor Anichebe's shirt in the penalty area.
Dorrans stepped up and sent Kiwi goalkeeper Jake Gleeson the wrong way from the spot before almost doubling the lead soon after.
Youssouf Mulumbu's pass from the right was well brought down by the Scot, who fired straight at Gleeson from the edge of the area.
West Brom struggled to keep possession after opening the scoring and the Republic came into the encounter more, with Alvarez particularly dangerous.
Saido Berahino almost made it 2-0 early in the second half but he was denied in a one-on-one by substitute goalkeeper Dominik Jakubek.
Myhill saved from Thomas Stewart in two one-on-ones and West Brom held on despite having Gardner sent off for picking up a second yellow card, with another meeting against Sacramento set for Friday.
