Old Firm referee Willie Collum received a number of threatening phone calls after Sunday's Glasgow derby at Parkhead, the SFA have confirmed.[LNB] The 31-year-old made his debut in the fixture and was criticised by Celtic manager Neil Lennon for awarding Rangers a penalty in the second half of the 3-1 win for the visitors.[LNB]The official reportedly answered a number of phone-calls at his Lanarkshire home with the caller threatening to "go after him and his wife and children".[LNB]It is understood that Collum reported the calls to the police but turned down the offer of protection outside his home.[LNB]SFA spokesman Darryl Broadfoot told the Daily Record: "Willie received a number of threatening calls at his home on Sunday night, one of which was taken by his wife.[LNB]"This kind of behaviour is abhorrent and has no place in football."[LNB]The incident can only de-motivate further the several top officials who are believed to be considering their futures due to the intense scrutiny they are being placed under.[LNB]Assistant referee Steven Craven resigned on Monday after being caught up in a furore over a decision to deny Celtic a penalty at Dundee United last week.[LNB]Meanwhile, Celtic assistant manager Johan Mjallby hit out at his players' performance against Rangers, telling the Scottish Sun: "I was disappointed with how we lost the initiative in the second half.[LNB]"It was almost like boys against men in the second half.[LNB]"Rangers were very effective, but we did not get a shot on target in the second half."[LNB]Mjallby, however, refused to single out any of his players, saying: "We do not comment on individuals.[LNB]"We win and lose as a team, but we gave away goals - and the match - too cheaply."