The former Chelsea midfielder twice scored in FA Cup wins at Wembley for the Blues but could not find the inspiration to prevent his West Brom side from slipping to a sixth straight defeat.
Long made it seven goals in six games with a cool finish before half-time while Albion substitute Jonas Olsson was booked twice within 15 minutes of coming on at the break to add to the Barclays Premier League side's woes.
Albion are already shorn of suspended centre-half Gabriel Tamas and Di Matteo must surely be weighing up a move for more defenders this month after the deficiencies which were evident in defeat at Fulham on Tuesday shone through again.
Even a change of goalkeeper could not save Albion. Boaz Myhill was handed a rare start after Scott Carson's costly blunder at Craven Cottage in midweek and was greeted with chants of "Albion's number one" from the visiting supporters after a stunning early save to deny Noel Hunt.
But the problems at the back were again ruthlessly exposed in the npower Championship side's devastatingly simple strike four minutes before the break as Noel Hunt won a flick-on and Long was able to cruise past stand-in centre-half Gianni Zuiverloon and steer beyond Myhill.