Frank Lampard spared Kepa Arrizabalaga from criticism and said he has no doubt Timo Werner will come good after Chelsea's unconvincing FA Cup win over Luton Town.

Tammy Abraham's hat-trick was ultimately enough for Chelsea to overcome Championship side Luton 3-1 on Sunday and take their place in the fifth round.

A mistake at 2-0 up from goalkeeper Kepa – who has been demoted to second choice following Edouard Mendy's arrival at Stamford Bridge – enabled Jordan Clark to pull one back for Luton.

However, Kepa made amends with a fine save from Harry Cornick early in the second half, with Abraham's tap-in then rounding off the win.

Werner, who has scored just once in 16 games across all competitions, missed a late penalty as his frustrating run in front of goal continued.

Since he last scored in the league – on November 7 – Werner has attempted 21 shots, with just seven hitting the target, while the Germany international has missed seven big chances.

Lampard, however, stood by both Kepa and Werner, insisting the goalkeeper was not solely to blame for Luton's goal, while reiterating his confidence in Chelsea's misfiring striker.

"Kepa makes a really top save in the second half to make sure it doesn't go to 2-2," Lampard told a news conference.

"It is very easy for people to sort of hone in on that but Kepa is not playing so regularly. He comes in after training well and he is a good goalkeeper and I will certainly give him that one in the first half.

"I am as annoyed that we let someone stand in our box in space and get a shot away when we shouldn't have done. I think it was a general team moment of complacency with how we were at 2-0 which is not good enough.

"So, we got back to it very quickly and dominated the game and it made us slightly nervous to not have the two-goal lead that makes it comfortable. Once we got that back again, which we should have got a lot earlier, then the game was comfortable.

"With Timo, I'm disappointed for him. He gave a lot to the performance today. He was a threat, his link-up with Tammy, with people around him, it's just one of those things.

"When it's not going for you, it can happen. I missed penalties, I know the feeling. I know he will feel despondent, maybe now and the last few minutes of the game, but he shouldn't be.

"It's my job and his team-mates' job to lift him and he will be fine. It was almost a sign of what's going on for Timo at the minute, it's not quite dropping for him.

"But there will be a time when he takes penalties and hits the back of the net without thinking about it. At the moment that's not quite the time for Timo, but it will come and hopefully soon."

Chelsea lost 2-0 to Leicester City in the league on Tuesday, but have now won 52 of their last 54 FA Cup ties against sides from a lower division, with Barnsley up next in the competition.

"I always need the results," said Lampard, who has been under mounting scrutiny. "People expect a lot from our league position.

"People will look at it and look very closely at the moment but I look at the bigger picture. Of course, I wanted the result today. It is a nice air of positivity around the club to get to the next round. We are in two cup competitions with the Champions League, FA Cup, and we want to win more points in the league.

"That's a good three wins in four games, I'm happy to quote that one because in the modern day people will very much look at the short-term stats that can be flipped in different directions.

"It's nice for us in the short term that we have done that. The scrutiny on a club like Chelsea is normal. The job for us is not to get too caught up in how many games we won recently or where we are in the table."