RUBEN AMORIM’S week has gone from bad to worse after an injury blow to summer signing Matheus Cunha.Cunha, who arrived at Old Trafford for £62.5million earlier in the window, was forced off with an injury during the first half of United’s clash with Burnley.AFPMatheus Cunha was substituted in the first half after picking up an injury against Burnley[/caption]Ruben Amorim could lose Cunha for some crucial fixturesPAThe 26-year-old trudged down the tunnel after just over half an hour had been played, much to the frustration of the home support.Cunha, who has looked like one of United’s best players since his arrival from Wolves, appeared to be clutching his hamstring when he was down on the pitch.Although the severity of the issue is yet to be determined, the Brazilian could now be a doubt for United’s huge derby face-off with Man City after the international break.United will make the short trip to the Etihad to take on Pep Guardiola‘s side on September 14.For Cunha, it could also mean that he is forced to withdraw from Carlo Ancelotti‘s Brazil squad to face Chile and Bolivia in the coming weeks.United supporters weren’t just frustrated to lose Cunha, but also left baffled with Amorim’s decision for his replacement off the bench.Amorim opted to bring Joshua Zirkzee on in his place, snubbing new £74m striker Benjamin Sesko.Sesko, who is yet to start a league game for United, seemed the obvious choice to replace Cunha – leaving supporters venting their anger on social media.BEST FREE BETS AND BETTING SIGN UP OFFERSOne wrote: “Don’t understand why Sesko wasn’t bought on for Cunha, bizarre decision.”While another added: “Amorim going for Zirkzee over Sesko to replace Cunha is just unbelievable.” A third asked: “Does Amorim not fancy Sesko as his No.9 if Zirkzee is coming on ahead of him after Cunha is injured, or is he just not fit enough yet?”And a fourth fumed: “Sesko is already third choice striker”.It comes after Sesko was left to take the TENTH penalty in United’s humiliating Carabao Cup exit to League Two side Grimsby in midweek.AFPJoshua Zirkzee was brought on for Cunha[/caption]EPABig money signing Benjamin Sesko was snubbed by Amorim[/caption]