The England international has been sidelined for the past five months with a knee injury, but played 45 minutes for the under-21s on Thursday night.
Arsène Wenger says the 20-year-old could be in contention to play at Aston Villa on Monday and Mertesacker is looking forward to having him back.
We need him because he can always make the difference with his speed, with his passing and obviously with his finishingPer Mertesacker
“We need every single player and in particular Alex,” he told Arsenal Player. “It was a real shock when he was out after Aston Villa.
“Following that game we did really well as a team but we need him because he can always make the difference with his speed, with his passing and obviously with his finishing. He can [make] absolutely a great impact for any team in the world so we are really pleased that he's back.
“But I think he needs some time to get through training and to have some good games, but I’m really pleased for him that he got 45 minutes against Fulham. That is a step back to absolutely full fitness.”
Mertesacker is pleased with how well-placed Arsenal are at present, and says the balance between youth and experience gives them a real shot at success.
“Experienced players are always good but we need to stay in the right shape and in full fitness because with age you are more vulnerable to injuries and to things like that,” he said.
“We need to keep the focus high on our health and on the young talented players as well. But I think we have a great balance in the team and a good mixture, so if we keep that concentration high we have a good chance.”Copyright 2014 The Arsenal Football Club plc. Permission to use quotations from this article is granted subject to appropriate credit being given to www.arsenal.com as the source 10 Jan 2014