Aston Martin's 2021 car is "not that good", according to rival Alpha Tauri's team boss Franz Tost.

Sebastian Vettel joining the former Racing Point outfit after being ousted by Ferrari had been billed as a return to a happy environment for the beleaguered quadruple world champion to thrive in.

But Vettel struggled massively in Bahrain.

"I don't feel at home in the car. Many things are fighting against me so that I cannot concentrate on the driving," the 33-year-old told RTL after the 2021 season opener.

"There are a lot of things that we have to take care of," he added. "Some of the others were two seconds faster."

After the 'pink Mercedes' controversy of last year and Sergio Perez's victory in that car, many predicted that Vettel might even be a dark horse for a fifth title in 2021.

"Theory is one thing, reality is another," Alpha Tauri boss Tost told Sport1's AvD Motor und Sport Magazin.

"In reality, the car is not that good," the Austrian charged. "It is very unstable at the rear end."

However, Tost thinks Vettel still has time to work his way back up to his former speed.

"With his experience, he can bring a team forward. He knows how to win races and world championships," he said.

Even Fernando Alonso, who fared significantly better than Vettel despite his two-year absence from Formula 1, says he is still slowly readjusting to life at F1 speeds.

"I think all the drivers who have changed teams - Carlos (Sainz), (Daniel) Ricciardo, (Sebastian) Vettel - need more time," he told the broadcaster DAZN.

"Vettel has to get used to it, Ricciardo is getting used to the brakes, Carlos in some races will not be at 100 percent, so imagine that multiplied by two and a half years away," Alonso added.