Saracens director of rugby Mark McCall welcomed a successful comeback by Billy Vunipola as Bristol endured a 44-23 Gallagher Premiership defeat at Allianz Park.

Vunipola played 36 minutes after appearing as a third-quarter replacement and made some telling contributions, including a forceful try upon his return from the refractured arm sustained against South Africa in June.

The England number eight has been blighted by injury for the past two seasons and McCall hopes his bad luck has been exhausted.

"It was good to get Billy back and he looked OK. His arm is fixed and he will only get better the more he plays now," McCall said.

"He hasn't played a lot of rugby over the last 18 months and hopefully he can get some consistent game time."

Wales wing Liam Williams ran-in a hat-trick despite only coming on as a replacement upon his return from a groin injury, but overall McCall was unhappy with the performance.

"Liam Williams looked OK and we were able to score a few tries down the wings," McCall said.

"We were disappointed with our first-half performance. They did a brilliant job on us at the breakdown. We weren't aggressive or urgent enough.

"To give the players some credit our second-half performance was way better, albeit against 14 men. But for us that performance isn't good enough. We need to be better."

A turning point arrived in the 48th minute when Australia flanker George Smith was sent off for a high tackle on opposite number Jackson Wray, who subsequently failed a head injury assessment.

It was a harsh call made when the score was level at 18-18 following an impressive first half from Bristol.

"If you put it into context of other decisions, I don't think it's any more than a yellow card," Bristol head coach Pat Lam said.

"It's certainly a penalty - without a doubt. A yellow card? Possibly. It needs to be clear and obvious but the number of replays they used suggested it wasn't clear and obvious.

"When you come to the champions and play on that pitch, playing with 14 men is always going to be very difficult. We were right in the game at that stage.

"We chased the win, even with 10 minutes to go. We were hunting for the bonus point right until the end. We came up short, but not for the want of trying."