Paul Merson says it is "worrying" that Chelsea manager Alonso has yet to settle on a stable backline, and if he doesn't fix it soon, it could become a huge "problem."

Chelsea started the Premier League season with two consecutive wins over Fulham and Brighton, but then lost 2-1 to rivals Arsenal away from home after taking an early lead and conceding a comeback.

Last weekend, the Blues suffered another disappointing setback, drawing 2-2 at home to newly promoted Hull City. Chelsea have yet to keep a clean sheet in the Premier League this season.

Merson said: "The problem is that after four league games and two cup games, Alonso still doesn't know his ideal defensive setup. He's not sure whether to use three at the back, five at the back, or four at the back."

"This is a hidden danger, and he must sort it out."

Maxence Lacroix did not start against Hull City and only came on as a substitute late in the game. If Alonso plans to play a three-centre-back system, he must establish a solid trio of centre-backs.

"Once these three centre-backs are chosen, they must play together for four or five consecutive games."

Merson believes that Alonso must establish a "solid" defensive foundation if he wants to lead Chelsea back to challenging for the title.

"It won't get worse indefinitely; they won't concede two goals every game," he added.

"They must stabilize the defense. Chelsea certainly don't want to wait until December and still be discussing how Alonso can't find the right balance in defense.

This is worrying, and they must resolve it as soon as possible."

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