"Winning the Lions series was a massive feat for the Springboks," du Plessis told South Africa's Business Day after Peter de Villiers ' men won the first and second Tests in Durban and Pretoria.
"It was a fantastic win and it was good for our rugby."
However, du Plessis is not convinced that his countrymen were at their best during the three-match series - a series in which the Lions could have won the opening international and perhaps should have been victorious in the second.
"It is debatable whether the Springboks played their best rugby," added the 49-year-old.
"I think people would have loved to see them register a whitewash, but it was still a great series.
"It was very intense and physical and it tested our defensive systems and support play.
"At times, we had good quality first phases. But our rucking could have been more effective. Our attacking play was good, but our execution let us down. Looking forward, we need to improve those areas.
"We also need to exercise caution when changing settled combinations. It is vital to manage the way players are substituted and the way we change the starting line-up."
Having seen his '97 squad score more tries than the Lions last time out but still lose the series, du Plessis has sympathy for the Lions after the roles were reversed this time around.
Du Plessis was impressed with the standard of rugby produced by Ian McGeechan's men and felt that their chances of a series win would have increased if the tour had included more than just six warm-up matches.
"The Lions did well, they played quality rugby. In the last Test, particularly, they converted their opportunities into points at crucial times.
"In the first two Tests the breakdowns were a mixed bag. But their defensive system was effective, which put pressure on us at times.
"The Lions have limited time to form player combinations and to get the squad to gel as a unit because they include players from four countries, which is difficult. I think if the tour was longer they would have performed even better because they would have had sufficient time to develop star combinations."