And he reckons there will be few players ever to have played in Scotland with a CV as impressive as his countryman's.
Numan struck up a wonderful understanding with Zenden at Dutch side PSV Eindhoven when they spent four years together as a double-act on the left-hand side of the pitch.
They forced their way into the Holland international team to star at the World Cup in 1998, before earning big-money moves abroad that summer when Numan agreed to join Dick Advocaat's revolution at Rangers and Zenden joined another Dutch colony at Spanish giants Barcelona.
Although Numan ended up spending the rest of his career at the Ibrox club, his former team-mate moved to the English Premier League to play for Chelsea, Middlesbrough and Liverpool before starring in France for Marseille.
His most recent destination was Sunderland last season, but Zenden has spent the last week training at Lennoxtown under the watchful eye of Celtic manager Neil Lennon as he runs the rule over the 35-year-old midfielder with a view to signing him for the remainder of the season.
And, according to Numan, the Parkhead club would be getting their hands on a player with a tremendous amount of top-level experience.
"There won't be many players with the CV Bolo has," said Numan. "He won over 50 caps for Holland and you have to remember some of the great players he was competing with at that time. And, of course, the clubs he played for.
"You don't play for PSV, Barcelona, Chelsea, Liverpool and Marseille without being a tremendous talent and he has enjoyed an incredible career. He has played in World Cup Finals, at the European Championships and in three of the biggest leagues in world football.
"I don't think there would be many players to have played in Scotland with an impressive a career as Bolo and that is an indication of just how good a footballer he is. I actually read in the newspapers here in Holland the other day that he was training at Celtic.
"I know he didn't play that much last season at Sunderland, but he was always extremely fit and his main asset was his pace. He was very quick and that made it really difficult his opponents. But he is a very intelligent footballer."
He went on: "The good thing about him is he can play in a number of positions. He can play left-back, left midfield, central midfield or even just behind the striker. I loved playing in the same team as him because I always knew where he wanted the ball and in what direction he would move."
Numan arrived in Glasgow alongside countrymen Giovanni van Bronckhurst, Michael Mols, Bert Konterman and brothers Frank and Ronald de Boer.
At the time, their arrivals raised the bar in terms of the quality of player operating in Scotland and it was a major factor in Rangers enjoying a great deal of success under Dick Advocaat, until Martin O'Neill then arrived at Celtic and began to turn the tide.
Although the current financial climate makes it impossible to bring in men of that calibre at the peak of their powers, the two clubs are still in a position to provide a big enough incentive to tempt the odd top-class talent to the city.
"It is tougher for all the teams, but especially Rangers and Celtic because they have to try and sign quality players for their supporters," added Numan.
"When I left PSV to join Rangers, we signed a lot of experience players and we had a really, really strong team. There was obviously a lot of Dutch players because Dick Advocaat took over and that was a big factor, but Pierre van Hooijdonk was a fantastic player for Celtic as well prior to that.
"I think the people in Holland respect Scottish football and if Bolo signs for Celtic then it will attract a lot of attention over here. Obviously I remain a Rangers supporter, but if I was to give him any advice then I would say sign for Celtic because they are a huge club.
"The atmosphere when you play for the Old Firm, especially in the home games, is fantastic and it would be a great way for him to end a wonderful career because of the huge clubs he has played for in the past."