Campbell, who made his 500th Premier League appearance this weekend, told ESPNsoccernet: "Chris is such a lovely guy. Top man. This will hit the players hard. The players admired him, and liked him, and won't be happy now he's gone like this."
The former England, Arsenal and Tottenham centre-back added: "You have got to ask yourself why this has happened because it makes no sense. Here is a guy who has done an unbelievable job. He got the club back into the Premier league and any manager would have been rewarded for that with a new contract - but Chris wasn't.
"The players are sure to be asking themselves 'what the hell is going on?', that is only natural.
"We all had a good relationship with the players, and when we left training this morning none of the players had a clue this was coming.
"But word started to spread that the bookies had stopped taking bets and the players started texting each other with the news.
"Yes, it has come as a shock, and it will affect the dressing room there is no doubt about that."