Both. Rashford should be utilized as a shadow striker. Jose has already made it clear that he doesn't see Rooney as a midfielder ("Maybe he's not a striker any more. Maybe he is not a No 9 anymore but he will never, with me, be a No.6. He will never be 50 metres from the goal. For me he will be a No 9 or a No 10 or a nine-and-a-half, but with me he will never be a No 6 or even a No 8." ), and therefore Rooney whilst occupying the no.10 role isn't responsible for being a traditional playmaker, but rather as a goal scorer. Similarly, Jari Litmanen had the role of a shadow striker for Ajax respectively, as did Muller for Bayern Munich under Juup Heynckes' 4-2-3-1 which is comfortably one of the most prominent 4-2-3-1 system in recent years. Speaking of Muller, Jose has already confirmed that Muller was very much a player he was interested in, as exemplified by "is anyone in England interested in Thomas Muller? For sure. Can we, England bring Thomas Muller from Bayern Munich? I don't think so." This suggests that Jose was open to having a shadow striker at Chelsea, and it's widely believed by many that Jose wanted Griezmann, another whose mostly suited best as a shadow striker, and the Atletico Madrid president Enrique Cerezo said the following, "right now, Griezmann's thoughts are not on a sheik or blue princess coming to sign him." I felt it was particularly noteworthy that he mentioned "blue princess" which I assume was hinting at Chelsea. Furthermore, Rashford's link up play, is one of his best attributes, he offers pace from in behind which is an attribute Zlatan doesn't particularly possess, and the "almost like a free role" as Rooney put it would suit Rashford quite well due to his exuberance he conveys each time he steps on the pitch. Watch the first 3 minutes so of the following video, to see a brief visual representation of what Rashford is capable of when he isn't scoring.
Watch first 3 minutes:
At Man Utd, he could play as a shadow striker, the role Griezmann flourished in, playing alongside Giroud from the last 16 andthe subsequent games which followed, or the relationship Dybala flourished in with the big, powerful Mandzukic. Rashford can be our Griezmann or Dybala and Zlatan can have a somewhat similar role to that of Manzukic/Giroud.
Zlatan at PSG played in a 4-3-3 system which had no no.10 which allowed him to drop deep, as his current evolution as a footballer has dictated that nowadays he operates as a false no.9 which makes it paramount for him to have willing runners, and pace around him. At PSG he had that with Di Maria, Lucas Moura and even Cavani who whilst not being particularly fast, has fantastic off the ball movement. Having Rashford as the shadow striker, would mean we still don't have a traditional playmaker in the no.10 spot that predominantly creates chances, and therefore the aforementioned too can work well in conjunction, as when Zlatan invariably drops deep, Rashford can run beyond him. We'd also have the option of having Zlatan occupying two center halves due to his overwhelming physical presence, concurrently, Rashford instills fear in the opposition due to him being very direct.