Scarlets coach Phil Davies says the eight-times capped second-row is a welcome addition as Llanelli begin a tough run with Saturday's trip to Sale followed by tough home and away games with Toulouse and three Welsh derbies in a week over Christmas.
Davies said: "Scott was going well for us before he got injured and he is a real line-out specialist: he will add some more strength there for us. His work in the air is one of his major strengths and it is good to have him back at a crucial time with these crucial six or seven games coming up."
MacLeod will partner Wales international Vernon Cooper in the second row, but Davies has rested most of his Welsh internationals with only centre Gavin Evans on the bench.
Llanelli must beat Sale to have any chance of reaching the semi-finals, but the home side have been decimated by injury with British Lions trio Charlie Hodgson, Andrew Sheridan and Jason White all ruled out for the rest of the season with injury.