Lehto missed the first two grands prix of the season after an accident in testing and in four races since his return his best finish has been sixth, while Michael Schumacher, the team's No 1 driver, has won five out of six races.
Verstappen, 22, the team's official test driver, will replace Lehto until the Finn recovers full fitness. Verstappen took over from Lehto for the races in Brazil and Spain, qualifying ninth and 10th respectively, but failed to complete either race.
The contrast between Lehto's performances and those of Schumacher have been marked, and Maria Ballenca, of Benetton, said that Lehto had been struggling to match his team-mate.
She said: 'JJ has had problems with his physical condition, and had difficulties in getting close to Michael.
'At Monaco, JJ experienced great physical difficulties, and discussed with the team management the possibility of taking some time off in order to make a full recovery. At the time both parties concluded it was best for him to continue.
'He's going to take his time to get back to optimum fitness in order to perform at the level of which he is capable, which hasn't been the case so far this season.
'We have a great deal of faith in Jos. In the meantime, JJ will be testing extensively for the team. We strongly support him, and wish to see him back in the driver's seat in the very near future.'