Manchester United's Alexis Sanchez is expected to be out for a month after picking up a hamstring injury in training on Thursday.

Sanchez, who has struggled for form since joining the Red Devils in January, will almost certainly miss Wednesday's game against his old club, Arsenal.

United could also be without the 29-year-old for important games against Valencia in the Champions League and Liverpool in the Premier League.

In his press conference on Friday ahead of the weekend's action, manager Jose Mourinho said he thinks Sanchez will be out for "a long time".

"No scans yet but the player is experienced, the player knows what the injury is," the Portuguese said, per Sky Sports.

"And also, from the top of my experience, the painful scream, and the way the injury happened, I know that it is going to be for a long time.

"It is not a little muscular injury that, in one week or 10 days, the player is ready. So, I think Alexis has what I used to call an aggressive muscular injury.

"For me and also Alexis, the feeling before the scans is that the grade is different than the (Victor) Lindelof one, and the Lindelof one was not an easy one."

Sanchez reacted to his long-term injury by posting a picture on his Instagram story of himself lying on the sofa next to one of his dogs.

However, as sad as it is to see a player injured, football fans have noticed something truly bizarre about the picture Sanchez posted. See if you can spot it below.

Everyone knows Sanchez loves nothing more in the world than his two dogs, Atom and Humber, and it appears he has two identical photos of them on his mantle.

In the background you can see two photo frames, one black and one white, right next to each other and both with the exact same image of the Chilean cuddling his two dogs.

It hasn't taken long for football fans to notice and, naturally, they think it's very strange. Check out the reaction below.

There could be a good reason for Sanchez having two identical photos of his dogs side by side, but that doesn't make it any less odd. At least spread them out, Alexis.