City won the Premier League fixture 1-0 at the Etihad Stadium thanks to Edin Dzeko's first-half volley, but they missed countless chances to extend their lead.

Carlos Tevez and James Milner were denied by the woodwork, while Jack Rodwell also wasted several opportunities.

Tevez, Dzeko and Gareth Barry also went close, while the poor finishing was not limited to City, as West Brom were incredibly wasteful in a match they really should have taken at least one point from.

"West Brom played very well," Mancini told ESPN. "They probably started better than us for the first 10 minutes, but after that we missed so many chances.

"I am frustrated because sometimes it is impossible to be missing all these chances. When you have six or seven clear chances to score, you should score.

"It was a difficult game but we knew this before the game because we changed nine (eight) players.

"I'm pleased because they put in on the pitch everything. It was difficult for Micah (Richards), Jack (Rodwell), they didn't play for a long time."

Mancini's eight changes came in the build-up to Saturday's FA Cup final against Wigan at Wembley.

Wigan slumped to a 3-2 Premier League defeat against Swansea City on Tuesday to leave them three points adrift of safety with two matches left, but Mancini – who believes they can still survive relegation – is expecting a tough match.

"I am very sorry for (Wigan boss Roberto) Martinez but I think they have a chance to stay because they have two more games and they are used to playing for relegation," he said.

"For the final, I think it is the final… it is not important for the Premier League and I think they will be very strong."

Manchester City defender Richards admitted his side were below-par against a good West Brom side but added that he was pleased they won, a result that assures their berth in the group stages of the UEFA Champions League next season.

"I wouldn't say it was a great performance but we got the job done in the end," Richards said.

"West Brom came here and played some good football…they are already safe and playing with no fear and showed some good stuff tonight.

"We always want to play Champions League football. Chelsea are breathing down our necks (for second position), they won at (Manchester) United…we needed to win tonight."