HE has watched his side frustrate both Arsenal and Manchester City already this season, but Middlesbrough head coach Aitor Karanka has warned his players they are about to face their toughest test as they prepare to take on “title favourites” Chelsea tomorrow.

Chelsea’s players travel to the Riverside on the back of a remarkable run that has seen them win their last five league games in a row, scoring 16 goals in the process without a single concession.

A 5-0 thumping of Everton prior to the international break saw Antonio Conte’s side climb to second position in the table, but while they remain a point behind Liverpool, Karanka regards them as likelier title winners in the long run.

He feels they have a more rounded squad than their rivals from Anfield, and also regards Chelsea’s lack of European involvement as a key advantage over the likes of Arsenal, Manchester City and Tottenham.

“I have been impressed with them,” said Karanka. “Their manager has settled into the league really well, and within a few months he has his team second in the table and showing they are a really consistent team. They are scoring goals, and it is difficult to score against them.

“I think they are the favourites to win the league because they can take advantage of not playing midweek games in the Europa League or Champions League. In the future, I am sure they will get a benefit from not playing those games.

“They don’t have midweek games and that is really important. If you look at our last game against Manchester City, I was asked in the press whether they might have been tired because they had played against Barcelona. We are in November, and those questions are being raised.

“If you look at the last four or five seasons with Chelsea, they were in the same position, but now it is different for them. We are only in November, but some teams are already having problems from a fitness point of view, so imagine what they will be like in February, March or April. They have a really good squad, a really good coach, and they also have that advantage.”

They also have the leading scorer in the Premier League in the shape of the controversial and enigmatic Diego Costa.

The Spaniard might not be to everyone’s liking with his antagonistic attitude, spiky approach to opponents and tendency to sulk if things are not going his way, but his effectiveness in front of goal can hardly be questioned.

Karanka first encountered him when Costa was an Atletico Madrid player and he was the assistant boss across the Spanish capital at Real, and he claims the striker’s off-field persona could hardly be more different to the attitude that prevails when he is on the field.

Nevertheless, he regards Costa as one of the leading strikers in the world, and would readily welcome him into his own squad.

“He is Diego, and I don’t know how many years I have known him for now,” he said. “When I was at Real Madrid and he was at Atletico Madrid, he was exactly the same.

“He is a nice lad. He is completely different when he is outside the pitch, but when he is on the pitch, he wants to score and he plays the way he plays. Everybody knows him, and he is the kind of player that everybody would like to have in their team.”

This season, Costa has been playing in a remodelled Chelsea side that has seen Conte adopt a rarely-used 3-4-3 system with Marcos Alonso and Victor Moses playing as wing-backs.

The tactical switch has enhanced Chelsea’s defensive solidity, and also provided more attacking freedom to the likes of Eden Hazard, Pedro and Willian.

Karanka has been working on a number of tactical tweaks of his own in an attempt to counter Chelsea’s approach, but admits it is difficult to counteract a formation that is such a rarity in the English game.

“It is difficult to prepare for because there are not many teams playing in that way,” he said. “We will have to adapt to play against that shape, but we have players who can do that. I have been working in the last two weeks to try to do something against them.

“Other clubs might try to copy them, but it depends. I like that shape because I won a Champions League playing with that shape, but you have to have the right players, and you have to have the right time to work with them. It is a shape that I like a lot, but I am not using. I prefer at the moment to use another shape, but I know it is difficult to play against teams that play in that way.”