He would be a brutal match with Horford. Unplayable.
Johnson is just about unplayable with Horford though anyway. At least Lopez can play on one side of the court at a high level (unlike Johnson).
And yet, the Celtics are +5.9 points per 100 possessions when those two are on the court at the same time, the third-best pairing for Horford behind Smart and, interestingly, Rozier. It's only the 5th-best pairing for Amir, but in either case the numbers don't back it up as being an unplayable combo.