Krul will have the injury assessed by United's medical team before any decision is taken over his availability for next Monday's Premier League trip to Everton.
"Steve was at Newcastle when I was there - and that's quite a while ago," Ferdinand told the Evening Chronicle.
"He is a very good keeper and, even though Tim Krul has been the first-choice for a while now, Harps won't let anyone down if he is asked to come in.
"You need a couple of top-class goalkeepers at your club, and in Krul and Harps Newcastle certainly have that.
"I think everyone is waiting to see exactly what the injury to Krul is, and how serious it might be, but it is a shame for the lad.
"He has been in fantastic form for Newcastle over the last year
"I think he was probably goalkeeper of the season last year!"