Fears have grown over how long Krul will be out for after he picked up a problem while playing for the Netherlands on Saturday afternoon. His country beat Kazakhstan 2-1 but the victory came at a price for both the Dutch and the Magpies.
And Harper, a former Newcastle goalkeeper himself, has wished the 27-year-old all the best, while also joking about his and Stoke stopper Shay Given's 'dodgy knees.'
Stoke's Given, who was Newcastle no.1 for many years while Harper deputised, then responded - joking about a Harper comeback to cover for the injured Krul.
The Dutchman's injury, of course, leaves the St James' Park outfit in a troublesome position, with Karl Darlow currently out with a foot problem, as well.
That could lead Newcastle to promoting a youth goalkeeper or possibly recalling Freddie Woodman while their no.1 is out recovering from his knee issues.
Either that or Harper could get his old gloves ready, as Stoke man Given suggests!