Alexander made his first Clydesdale Bank Premier League appearance of the season in Saturday's 3-0 win over St Johnstone after McGregor suffered a hamstring injury.
However, the 31 year old is resigned to losing his place once his rival is fit - as he has done during his entire two-year Rangers career.
"It's extremely hard, especially at my age, because I want to play," said Alexander, whose contract expires in a year's time.
"And, having had a taste of playing at this club and the success I've had in a short period of time, it makes it twice as hard to be sitting on the sidelines and not involved when the boys are winning.
"If that was the case then I'd have to go away and think about it. I would need to look at my options and decide what to do.
"I love being at the club. I feel it's been the best time of my career, and my wife and kids are settled, so you have to weigh everything up involved with being here. We have to wait to see what the future brings, but there's only so long, I feel, before I need to get back playing. If it's not here, then maybe I need to look somewhere else.
"Do I have a time frame in my own head? Yes. I spoke to the gaffer after the first game of the season as I felt I needed to go in and have a chat with him. He said I had never let him down and that I've done brilliantly since I joined the club. He put his arm round me and said I was unlucky not to be in the team.
"But it's my career at the end of the day and, as much as I love it here and would love to stay for a long time, I feel I need to be playing. I've got an idea in my own head and we'll see what happens."