Double Formula One world champion Alonso has been on the podium five times in six races with four third places and a second in Monaco last weekend.
Stroll, son of the team's billionaire owner Lawrence, has failed to score in the last two races and also retired in Saudi Arabia in March when he was carrying wrist injuries.
He has scored 27 points to Alonso's 93 and while Aston Martin are second in the constructors' standings, they are only a point clear of Mercedes and need both drivers to be scoring points.
"I think Lance has been very, very unlucky the last two events," Alonso told reporters ahead of his home grand prix at the Circuit de Catalunya.
"He has provided the right feedback, info and strategy about everything.
"So I really hope for him that he has a little more luck because I don’t think that he’s missing any pace. It’s just opportunistic moments that were not on his side."
Stroll, whose seat is considered the most secure in the sport given the family connection, started the season on the back foot after breaking both wrists in a cycling accident in Spain.
He missed three days of pre-season testing and was seen struggling to extricate himself from the car after Friday's practice in Bahrain.
Stroll retired with an engine problem in Saudi Arabia, finished fourth in Australia and seventh in Azerbaijan.
In Miami, he failed to make it through the first phase of qualifying after a tyre miscalculation but finished 12th.
In Monaco, while Alonso started on the front row, Stroll qualified 14th after collecting debris in the second phase and retired from the race with crash damage.
"It’s luck, things happen in racing and we had some tough weekends, we had our retirement in Jeddah," Stroll told reporters separately.
"I think it’s just how it goes. There’s good weekends and bad weekends and like every other weekend I’m just coming into this one trying to get the most out of it."
