Manuel Lanzini is set to be out for two months after suffering a fractured collarbone during West Ham’s 3-0 defeat by Burnley on Saturday, heightening the growing pressure on Manuel Pellegrini.

Lanzini is due to have surgery on the injury on Monday and his hopes of playing again this year look slim. The Argentinian midfielder, who missed most of last season with a serious knee injury, required oxygen and was carried off on a stretcher after being fouled by Burnley’s Ashley Westwood during the closing stages at Turf Moor.

The problems are piling up for Pellegrini, whose side slumped into 16th place after yet another abysmal performance. West Ham are winless in their last six league matches and are five points above the bottom three.

While Pellegrini is not in any immediate danger of being sacked, there is concern at West Ham with the team’s slump. The board is said to be in shock, even though there is faith in Pellegrini’s ability to lead a revival. That belief could be tested when West Ham face Tottenham, Chelsea, Arsenal and Wolves after the international break.

West Ham have struggled in defence since losing their goalkeeper, Lukasz Fabianski, to a hip injury in September. Fabianski’s deputy, Roberto, was poor again against Burnley, performing nervously and scoring an own goal when he punched a corner into the West Ham net.

Roberto looks bereft during West Ham’s defeat at Burnley, in which he made a number of key errors. Photograph: Robbie Jay Barratt - AMA/Getty Images

There is unhappiness within the club at the decision to sign Roberto on a free transfer last summer. The 33-year-old Spaniard was identified by West Ham’s director of football, Mario Husillos, and the signing was given the green light by Pellegrini. It is understood that Pellegrini and Husillos wanted to sign Roberto in the summer of 2018 and make him West Ham’s No 1. However, Fabianski was signed for £7m instead and was excellent last season.

West Ham’s defensive failings have been exposed by the absence of Fabianski, who is not expected to play again in 2019. Burnley subjected Roberto to plenty of aerial pressure and the former Espanyol goalkeeper’s erratic performances have sapped the team’s confidence. Pellegrini has backed him but the manager faces calls to replace him with the former Millwall keeper, David Martin, when Spurs visit the London Stadium on 23 November.

It is hoped that Michail Antonio will be available to face Spurs. The forward has been out since August following an operation on a hamstring injury. However, Mark Noble could miss out after suffering a twisted ankle against Burnley.