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 on Saturday.
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 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 Championship side's devastatingly simple strike four minutes before the break.
Noel Hunt won a flick-on and Long was able to cruise past stand-in centre-half Gianni Zuiverloon and steer beyond Myhill.
Zuiverloon has spent most of the season on loan at fellow Championship side Ipswich and while he is an able full-back looks lost in the middle, while Olsson's cameo was disastrous.
Albion's attacking play did at least threaten occasionally, with Ishmael Miller twice going close on his first start of the season and James Morrison hitting the top of the crossbar, but there can be no doubt that Reading deserved their win.
Boss Brian McDermott led his side to Cup wins over Liverpool and Burnley last season and comfortably added another top-flight scalp.
Long has been linked with a move to West Ham in the transfer window and excelled, his movement too sharp for the visitors all afternoon as he added to his goal by being the recipient of both challenges which saw Olsson dismissed.
Olsson scythed through Long less than two minutes after coming on at the interval, and then saw red in the 60th minute after more bright play from Long.
A bad touch from the Swede was seized upon by Long and after the Republic of Ireland international nicked the ball away he was caught by Olsson's attempted challenge.
Referee Jon Moss had no option but to show a second yellow and leave Di Matteo with yet more problems at the back.
A poor last half hour followed with neither side creating much in front of goal as the Royals saw out a good day's work.
Di Matteo's thoughts will now turn to next weekend's home fixture with Blackpool and a chance to pick up the first league points since December 5.
For McDermott and Reading, round four awaits and the Royals showed enough to suggest that they have what it takes to mount a serious promotion challenge.