Roberto Soriano had put the visitors in front early in the second half but the midfielder's deflected overhead kick sealed a share of the spoils
Home supporters unveiled banners before Sunday's Serie A fixture urging owner Silvio Berlusconi to sell the club amid reported interest from Chinese and Thai investors.
And Milan's players failed to lift the mood on the pitch, requiring De Jong's late deflected equaliser to claim a point after Roberto Soriano had opened the scoring for Sampdoria just before the hour.
The result means Filippo Inzaghi's team sit seven points adrift of fifth-placed Fiorentina with eight games remaining and, to make matters worse, Mattia Destro was stretchered off with 13 minutes to play.
Sampdoria, meanwhile, are now level with Vincenzo Montella's Fiorentina.
Inzaghi named an unchanged side from the win at Palermo last time out and Milan appeared to benefit from that continuity during the early exchanges, confidently taking control in midfield.
Despite struggling to break through Sampdoria's solid backline, Milan posed a threat from range through Marco van Ginkel, the Dutchman first testing Emiliano Viviano with a 30-yard drive in the 14th minute, before striking just wide from a similar distance shortly after.
Milan eventually crafted a presentable opportunity inside the area just after the half-hour mark, but Destro could only head Ignazio Abate's right-wing cross straight at Viviano.
That pair combined again seven minutes later as Destro directed a tame half-volley at the goalkeeper under pressure from Matias Silvestre.
Milan's best chance arrived a minute before the break, but Alessio Cerci blasted over the crossbar after pouncing on a loose ball in plenty of space 20 yards out.
Sampdoria produced a marked improvement at the start of the second half and went close through Stefano Okaka within a minute of the restart, but he could only divert Djamel Mesbah's cross wide.
Eder saw a fine chance of his own go to waste seven minutes later, unleashing a powerful drive towards the bottom-left corner, which Diego Lopez tipped around the post.
The deadlock was eventually broken 58 minutes in, though, as Soriano coolly stroked past Lopez following an incisive Samuel Eto'o pass.
Milan responded well to that blow and drew level 16 minutes later, when De Jong produced a fine bicycle kick from 12 yards and his effort beat Viviano thanks to a helpful deflection off Alfred Duncan.
Destro was then carried from the field after an innocuous-looking challenge, but Suso almost completed a Milan turnaround seven minutes from time, only for the Spaniard's delightful curled effort to come back off the post.