West Ham can't take their eyes off Chelsea's Diego Costa - Winston Reid

Winston Reid has warned that West Ham's defenders won't be able to let Diego Costa out of their sight if they want to halt Chelsea's Premier League title charge at the London Stadium on Monday.

Antonio Conte's men have lost just one of 20 league matches since switching to a 3-4-3 system in late September and Costa has played a key role, contributing 11 goals and four assists in that time as well as avoiding the disciplinary problems that disrupted his previous two seasons in English football.

Costa was a second-half substitute when West Ham beat Chelsea 2-1 in a EFL Cup tie at the London Stadium in October but he is certain to start this time, and Reid believes he and his teammates will have to be at their best to stop the Spain international.

"When you play against top players you have to do your best and try and be at the best of your game or you get found out," Reid told the London Evening Standard.

"You have to try and keep your eyes on him at all times and make sure you're close to him and mark him in the box where he's dangerous."

Costa scored an 89th-minute winner for Chelsea when they beat West Ham 2-1 on the opening day of the Premier League season at Stamford Bridge, but Reid insists the Blues' tactical shift means the lessons West Ham learned that day will not be particularly relevant this time around.

"The first game was different because they played a different system," he added. "And the second game when we played them at home in the cup, they changed a couple of positions [in the 3-4-3].

"It's a different system, and they are doing really well. They are hard to score against but we put a couple of goals past them at home and we put one past them away. We'll be at home and hopefully we'll have a good crowd so we will have a go at them and see if we can win the game.

"They aren't leading the league for fun -- they've been scoring goals left, right and centre and conceding very few. It's going to be tough."