Krack insisted that Stroll remains the Plan A and that no alternatives would be considered until it is clear that the Canadian can’t drive in next week’s Bahrain GP.
Official reserve driver Felipe Drugovich remains the obvious favourite to get the seat given that he tested the car in Bahrain this week.
However, should Stroll's problems prove to be longer term and stretch to the second race in Saudi Arabia, Vettel would have more time to undertake proper preparations and, for example, try the AMR23 in the simulator. Vettel retired at the end of last season with 299 grand prix starts to his name.
Krack confirmed that he has spoken to the four-time world champion since news of Stroll’s cycling accident and wrist injury became public, although he declined to confirm whether or not the German showed an interest in returning to the cockpit.
“I have had a couple of phone calls with Sebastian,” said Krack when asked by Motorsport.com if they had spoken in recent days. “But this has been also last year, and this will continue in the future.”
Asked if Vettel had indicated an interest in driving, he replied: “I will not tell you.”
When asked if the longer lead time to the Saudi Arabian race would allow Vettel more preparation time, should Stroll not be fit, Krack appeared to leave the door open.
“I mean, this is very, very hypothetical,” he said. “First of all, our plan is to have Lance in the car. And then we continue to see because we have Bahrain first before we speak about Jeddah. We have not made the final decision. That's the point.
“Don't forget one thing. He had a very, very thorough plan in mind for his retirement. And, you know, I think this is also something that you have to respect. So let's see what's happening.”
Krack admitted that Stroll’s situation remains unclear: “I honestly don't know at this point. He’s our Plan A obviously, we want to have Lance back in the car, but we have to wait, what he says and what the doctors are saying in the right moment.
"There are certain delays or certain limits that you have to give to the FIA, and then we will respect this. We'll have to see how the situation is developing over the next day. It's like in football, you have players that are a little bit injured, and you wait for the last moment, and then you decide if they can play or not.
“The Plan B we have to decide. We have some Plan Bs, but we have to decide the Plan B when the Plan A is not possible. It's very simple. Can he drive or can he not drive?”
Krack confirmed said Stroll has not been in the Silverstone team's simulator, but the plan is for him to try it: "Not yet, he will go. It's the first step.”