Greig captained the club to their first and only European success, the 1972 Cup Winners’ Cup success over Dinamo Moscow.
Rangers will be looking to add a second European trophy on Wednesday night against Eintracht Frankfurt in Sevilla’s Ramon Sanchez Pizjuan Stadium.
It is estimated that more than 100,000 of the club's supporters have or will be making the trip to the Spanish city in the south of the country.
Legendary Greig, who won countless owners as player and manager of Rangers, will be attending the game and he wrote to fans to express the magnitude of the occasion, the gratitude for the support the team have received during the run to the final and fans they are ambassadors for the club and should look out for one another.
"I wanted to write to each and every one of you on the verge of one of the biggest matches in the club’s history,” he said.
“To see Rangers in a European final is something many of us thought would not be possible again, and I’ve been so taken by the incredible lengths you are going to in order to reach Seville to back the team out there.
“We have found ourselves on an incredible journey from Armenia to Andalucía in this tournament, and the coaching staff and every player who has played a part on that journey deserves enormous credit.
“50 years on from Barcelona, it is a great thrill for me to have been invited to join the team as once again we head to Spain trying to make yet more history.
“Of course, a great regret of mine was not being able to defend our trophy the following season in Europe. It is vital that we all act responsibly and remember the values that this institution stands for.
“Our 150th anniversary season has been a trying one, with the loss of Walter Smith earlier in the season, and then of Jimmy Bell a fortnight ago. Both will be forever remembered for their enormous contribution to the fabric and traditions of this club.
“I look forward to seeing you out in Seville, and to seeing the absolute best of our club on display. There is no other club in Europe who would travel in such numbers for a match like this, and every single Rangers supporter should remember they are an ambassador for our institution.
“This is a time to be enjoyed by each and every one of us, but it is also a time to look out for one another, and to protect the good name of Rangers.
"I trust every single one of you to have the time of your lives, and hopefully see the Europa League trophy come back to Ibrox.
“Thank you again for your magnificent support, and as well as wishing you a safe journey to
“Seville, I want to wish Gio, Tav and all the boys all the best for the match.”