Per Mertesacker admits Alexis Sanchez's two-month injury absence contributed to Arsenal's failure to maintain their Premier League title challenge.
Sanchez has been in fine form in recent weeks and took his goal tally to five in four games as he hit the match-winning double in Arsenal's 2-0 victory over West Brom on Thursday night.
The win moved them up to third as they look to secure a 20th consecutive season of Champions League football but they once again look set to miss out on the title after topping the table at the turn of the year.
Chile striker Sanchez sat out the vital Christmas and New Year period with a hamstring injury and has taken time to recover his best form, a factor Mertesacker believes is telling.
"There are a couple of things we missed, but he is obviously one," he said.
"When he was out, I think the statistics were not that bad for us, I think we played 10 games without him and won seven. But obviously if he is on the pitch, he creates space always for other players because he draws so much attention and that is the key to any team.
Live Ford Super Sunday
"He is a key member of our team and, if he is fully fit, he makes the difference and that is always the case. During the time he was out, I think we had quite a good record, but over the whole season normally we need him healthy.
"He is the type of player who makes the difference and, even when he is in Chile playing two qualifiers, he will be back on track in the right time when having just two days' rest in between. He is that type of player who gives everything to the team. Those team members are well appreciated."
Mertesacker came back into the Arsenal side as one of three changes for the clash with Albion, having been confined to the bench due to the form of Laurent Koscielny and Gabriel.
The Germany international and World Cup winner was disappointed to be dropped, but is looking to take the positives from it.
Asked if being in and out of the side had been difficult, Mertesacker replied: "That's team sport.
"Obviously you are disappointed, but I will never drop the respect for anyone here, even when I'm not playing. I'll keep working harder. It was a good time for me. If you change a little bit of your perspective, you see how other people feel.
""But we have got 25 players, everyone wants to play and obviously the boss has got a lot of headaches with his selection. With players coming back, it is going to be tight and good competition. That is what we play for."