I know that feeling when a career in professional football is reaching its latter stages. When the legs ache more than they should in a warm-up, when it seems like a young lad in training has got the measure of you, when you jog on to the pitch and find yourself asking that same question: what am I doing here?
Those last few years can also be wonderful, as you savour the last experiences of a job you love. It comes to us all, and it is a case of adapting your game and conserving your energy for the moments in a match when you can be at your most effective. It is a change that Steven Gerrard, at the age of 34, is having to make as his career winds down.
Being rested against Real Madrid will have been a new experience for him, although he was not the only first-choice Liverpool player left out. All through his career he has played in those big games as a matter of course, but as he gets older there will be more occasions when he has to be rested. The question for Brendan Rodgers is whether he has the players to give Gerrard that privilege.
Gerrard should have started on Tuesday. I recognise that Rodgers had a run of three games in a week but if he could not play Gerrard in all of them then it should have been last Saturday's game at Newcastle that he missed. The problem is that Gerrard does not have the players around him to allow his own playing time to be managed.
There is no reason that he cannot be an effective holding midfielder for Liverpool for three more years. I won a Champions League final six months before my 34th birthday. You need a manager who knows when to play you and when to rest you. Alex Ferguson was brilliant at that. He would tell me that he had a game in mind for me, a week, 10 days ahead and to get myself ready to play.
Liverpool's squad is not strong enough to allow Gerrard the rest he needs, even taking into account the performance in Madrid by a mainly second-string XI. In my last few years at United we had the strength in depth that allowed the manager to be selective with my games. But Rodgers clearly feels that he has no alternative with Gerrard.
Liverpool's Daniel Sturridge has suffered a fresh injury blow
Once again we go back to Liverpool's signings in the summer. They did not recruit well enough. They are relying on the same players as last season to bring them through the big games. Rodgers said as much before the Madrid game, that it was time for those who had not been playing to take responsibility.
Teams are only as good as their strikers and Liverpool's have not been good enough since they sold Luis Suarez and lost Daniel Sturridge to injury. A player like Gerrard thrives on having a striker who is always on the move, opening up angles with the intelligence of his running. When Sturridge comes back he will give them that.
Last season they came close to winning their first title in 24 years, and they played well. Now we keep hearing that they are in "transition". They are in transition because they did not buy well enough in the summer. I feel they needed to concentrate on signing one top striker, one top midfielder and one top centre-half. Instead they spread their net too wide.
I see Gerrard's last years at the club being spent in the quarterback position that he has occupied for most of the last two seasons. Rodgers has tried playing him behind the front line but I felt that was more because he was trying to solve Liverpool's failure to score goals rather than using the player to his greatest strengths.
As a player gets older there are adjustments that have to be made every season. When I retired from international football in 2004, I struggled at first to know what best to do with those days during international weeks when the rest of the United squad was away. Should I take the time to have a break? Or should I work through as usual? Like many players I liked to train every day, and did not always feel right if I didn't.
Ryan Giggs was the master at getting himself just right to play. He was meticulous about it. He always knew when to rest and when to train; when to train hard or when to ease off. Gerrard is in his first season without international football and he will be trying to find that balance.
There is no doubt that Gerrard has earned the right to be considered a great player, even if it now looks inevitable that he will never win a league title. As for the final few years, I cannot offer him much consolation. It is horrible getting older.
Brendan Rodgers insists his decision to play a different team against Real Madrid on Tuesday has the support of the Liverpool hierarchy. Gonzalo Arroyo Moreno/Getty Images
At United we used to play an eight-a-side game in training on a Friday and in my final couple of seasons there were times when the game passed me by. I found that the likes of Tom Cleverley were just passing the ball around me.
But there were still moments in games when all the pieces fell into place and I contributed in the way I had in my younger years. For that you need to be fresh and ready. Liverpool must find a way of ensuring Gerrard has sufficient support to feel that way for their biggest games - like that one in Madrid.