Rangers have won the Scottish Championship title this season and will feature in the Scottish Premiership during the 2016-17 campaign.
The Gers are likely to make signings in the summer transfer window in order to strengthen themselves and compete with Celtic.
Former Celtic striker Hartson believes that Rangers boss Warburton needs to make sure that the funds he gets in the summer are spent wisely.
“There simply must be realism into what happens behind the scenes in the next few years,” the former Wales international wrote in The Scottish Sun.
“That may mean Warburton only has £5m to £7m to spend this summer, but if he gets that he should take it and use it wisely. That would allow him to bring in four or five players who could make a big difference to their chances.”
Rangers have been quite good in signing players under Warburton, who joined the club in the summer of 2015.
The Englishman has proven to be very good in identifying the kind of players that would enhance the quality of his team.
The fans should remain optimistic of Warburton making the right additions to the squad for the 2016-17 Scottish Premiership campaign.
Rangers, who won the Scottish Challenge Cup on Sunday, will take on Celtic in the semi-finals of the Scottish Cup next weekend.