Centre-back Shawcross was given a red card for a professional foul onClint Dempsey, resulting in his second goal in a 2-0 win via a penalty,which ordinarily carries with it a one-match suspension.
However, as it was Shawcross' second red card of the season - he was sent off for two bookable offences at Sunderland in November - it means he will now miss two matches.
Penalty: Clint Dempsey is pulled down by Ryan Shawcross on Saturday
Shawcross sits out the FA Cup fourth-round tie at Wolves on Sunday, as well as the Barclays Premier League trip to Liverpool three days later.
Speaking to the club's official website, Pulis said: 'I was disappointed with the award of the penalty because from my point of view it was a coming together of two players.
'To me, Dempsey looks like he is already on his way down before Ryan puts his arm around him.
'Even if you accept that it was a penalty, for the referee to then send him off adds insult to injury.
'What is particularly harsh is that we now lose him for another two matches, so the punishment doesn't fit the crime, and that really needs looking at.'
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