The Light Blues were dismissed with ease in Sunday’s William Hill Scottish Cup semi-final at Hampden Park by the Hoops whose 2-0 win was no reflection of their superiority in a season where they have not lost in five Old Firm meetings.
The Parkhead club secured their sixth Ladbrokes Premiership title earlier in the month and are gunning for the treble in their first season with Brendan Rodgers as boss.
Henderson, star of the Ibrox club in the 1960s and early 1970s, was speaking at the national stadium where he was helping launch this year’s nominations for the Scottish Football Hall of Fame.
The former Scotland player was scathing of Rangers’ efforts on Sunday and said: “The fans at least expect an enormous effort from players wearing the blue jersey.
“They accept that Celtic are a better team just now but they shouldn’t be a better team with effort and that was disappointing for the fans who have put up with a lot for a long time.
“If Rangers are going to compete with Celtic, there has to be a huge investment in this club to purchase players who are going to be of a standard who can play for Glasgow Rangers and compete with Celtic.
“£50million, solely for players. It might be an idea for another investor, if there is another investor who could be brought into the club who is interested in putting money into football. It is certainly needed.
“I am not privy to anything that goes on in the business side of it but if it is going to take someone else to be added to what is already at Rangers, prepared to invest the money in the club, then maybe it is time that the board sit round the table and discuss it with other people.
“You are going to have the same situation that you have just now if they don’t do that.”
Henderson believes it is over half a century since there was such a gulf between the two clubs.
Ahead of the last league meeting of the season between the clubs at Ibrox on Saturday, he said: “We played Celtic on the Saturday (1963 Scottish Cup final) and drew and on the Wednesday we beat Celtic 3-0 in the replay, in front of a quarter of a million people in total.
“At that stage there was a huge gulf between Celtic and Rangers.
“We were far in front but then along came the genius Jock Stein (Celtic’s European Cup winning manager). The gulf today is the same as before Jock Stein came.
“We were always very confident of beating Celtic. Today, I don’t think there is a Celtic player who if they are going out and putting a shift in, would fear getting beat by Glasgow Rangers.”
Henderson believes that any manager, not only the recently appointed Portuguese coach Pedro Caixinha, would struggle with the Ibrox squad of whom he would keep only “three or four” for next season.
He said: “There are a lot of players at Ibrox who, for a considerable time, have had a chance to prove themselves to be of that class and I am afraid some of them are not that good.
“Kenny Miller has had a really good season. He would be the first to admit that he is getting on but with his experience I would keep him on,
“I would keep the goalkeeper (Wes Foderingham), Lee Wallace and the lad (Emerson) Hyndman.
“These are good players. But you would certainly need to add to the squad.
“It is all very well bringing managers in but as I said earlier, when Jock Stein came in, players like Jimmy Johnstone, Tommy Gemmell and Bobby Murdoch were there.
“We don’t have players of that calibre just now, for any manager coming in, to be able to improve the team that much.”