Koren refused to shake Di Matteo's hand when substituted in West Bromwich's previous game and was initially left out here.
Di Matteo called him 'selfish' but, during the warm-up, Koren was suddenly told he was playing and went on to score. It could have been even better as he also hit a post from 25 yards.
Early bird: Reading's Jimmy Kebe scores the opening goal
QUICK BUT NOT THE QUICKEST Timed at 9.7sec, Jimmy Kebe's goal for Reading was the fastest-ever in the FA Cup proper, but Gareth Morris's four-second strike for Ashton United in the qualifying round in 2001 is the competition's quickest.
'People know that I'm not selfish,' said Koren. 'I had opportunities to move on but, in the end, we decided that I will stay and try my best for the team and get us into the Premier League.'
Koren scored West Brom's first equaliser and Joe Mattock, 19, delivered their second with under five minutes left.
Reading scored after only nine seconds. Gianni Zuiverloon struggled to control a bobbling ball on a poor surface, leading to Jimmy Kebe dispossessing him and squeezing the ball past Scott Carson, but Albion responded in the 18th minute with Koren's effort.
Hitting back: Robert Koren (second left) celebrates scoring
Reading were in more trouble in the 48th minute as Shane Long was sent off for a senseless, studs-up challenge on Abdoulaye Meite. Long said: 'It was frustrating up front and the ref (Chris Foy) didn't help me out.'
Foy made his mark again when he harshly showed a second yellow card to Youssuf Mulumbu. With numerical parity restored, Reading brought on Grzegorz Rasiak. The Polish striker took advantage of a rare appearance by chipping the ball forward for Simon Church, who broke away and scored in the 72nd minute.
With Long suspended, Rasiak's prospects suddenly look rosier. He said: 'I just have to take my opportunities.'
That goal seemed to have sealed Reading's first quarter-final appearance in 83 years. Sadly for them, Mattock delivered the final twist.
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