"If they were to have a very good rest of May, June and July they could, but they will have to do that," the Scot told Reuters when asked whether the Ford-owned team could hang on to the 27-year-old.
Webber has scored Jaguar's only point of the year so far, despite qualifying on the front row in Malaysia, and is widely considered to be top of Williams' list to replace Colombian Juan Pablo Montoya for 2005.
He finished 12th in Sunday's Spanish Grand Prix. There are seven races to come before the end of July.
The Australian is under contract to Jaguar next year, although that is sure to be subject to performance clauses, but his manager and Renault team boss Flavio Briatore also has an option on him.
"I think he's one of the best five drivers in the world right now in Formula One," said Stewart, who sold his eponymous team to Ford in 1999 and is a non-executive board director of Jaguar Racing.
"Most of those drivers...have got immense gifts of natural, God-given talent and they just drive so fantastically well," added Stewart.
"But there's not that many of them who have got a head that goes with it. The mind management that he (Webber) has is probably his greatest asset."
Former champion Alan Jones, the Australian who won his title with Williams in 1980, told Australia's AAP news agency on Tuesday that Webber should leave Jaguar.
"I'd like to see him in a Williams or a Renault," he said. "Unless Jaguar has something up its sleeve, he should be out of there at 100 mph if he is given the chance."
