WATCH: Martin Skrtel gets sent off while lining out for Liverpool’s U21 side

It must be frustrating for Premier League footballers who are making their way back from injury.

Lengthy injury lay-offs usually lead managers to ease players back into the senior set-up, often letting them build up fitness for the under-21 team. That's precisely what is happening with Martin Skrtel, the Liverpool centre-half who was one of the victims of the hamstring crisis that plagued the Reds at the tail end of 2015. Skrtel has been lining out for the U21 side for the past few games in order to get back to fitness but he didn't make it through extra-time of the Premier League Cup semi-final against West Ham. The Slovakian was given his marching orders after he was penalised for a challenge on the edge of his area. He believed that there was minimal contact and made no secret about voicing his opinion as he charged at his opponent. The referee didn't hesitate to reach for the second yellow and Skrtel was sent strolling. West Ham proceeded to the final after beating the Reds 3-2 in extra time.