Chelsea’s uncharacteristically off-colour performance at Manchester United on Sunday has been put down partly to a bug that affected members of Antonio Conte’s squad and ruled Marco Alonso out of the game at Old Trafford.

Alonso was still suffering the effects of the virus on Saturday night and, despite being named in the starting lineup at United, complained of feeling ill after the pre-match warm-up. Chelsea’s medical staff assessed him in the away changing room and the wing-back was withdrawn from the team, with the league leaders going on to lose the match 2-0 to leave their advantage at the top trimmed to four points from Tottenham Hotspur with six games left to play.

Alonso’s late withdrawal forced César Azpilicueta to fill in on the left, with Kurt Zouma drafted in as a right-sided full-back and it clearly affected the balance of Conte’s team. Yet Alonso was not the only Chelsea player still suffering from the virus which had set in within the squad towards the end of last week.

One theory that has been considered by the club is that the bug had spread through the squad at a team bonding meal – one of a number arranged by Conte over the course of the season to strengthen the group – last Thursday evening.

Victor Moses, only just returned from a calf injury, had also been hampered and, having started the game, had to be withdrawn nine minutes into the second half prompting further disruption to the visitors’ approach. Diego Costa is understood to have been unwell and, like Moses and Alonso, had missed training at the back end of last week prior to his side’s trip to the north-west.

The striker declared himself fit but struggled to make much of an impact during the game, embroiled as he was from early on in tussles with Eric Bailly and Marcos Rojo in the hosts’ back-line. Costa, who was eventually booked for a foul on Paul Pogba, has failed to score in five consecutive Premier League games for the first time in his career in English football – his last club goal was at West Ham in the first week of March.While Conte has consistently praised the Spain international’s work rate over recent weeks, he appeared to lack his normal energy levels at times on Sunday.

The hope at Chelsea is the trio will be fit to play in the FA Cup semi-final against Spurs on Saturday, when the league leaders hope to inflict psychological damage on their nearest rivals in the title race. Yet the effects of the bug were still being felt at Cobham even on Tuesday when Gary Cahill reported unwell. Thibaut Courtois, who hurt his ankle while playing basketball at a Chelsea promotional shoot for the NBA last week, is hopeful he will have proved his fitness for the Cup tie at Wembley.