Salazar has struggled this year, making only two good starts (out of 8), averaging barely 5 innings a game, giving up too many home runs and too many walks, and taking too many pitches to get through even most of his good innings. His ERA stands at an ugly 5.53.
Kyle Crockett, a 22-year-old lefty. has been incredible since the Indians made him their 4th-round pick in last year's MLB draft. He's thrown 40.1 innings across all levels, and has allowed only 2 earned runs. That gives him an ERA of 0.45, and he's struck out 10.9 batters per 9 innings, while walking only 1.8 per 9 innings. Making it from the draft to the MLB level in under year is quite an accomplishment. He could be the Tribe's closer in another couple years.
It must be noted that Crockett is a reliever though, so while he's filling Salazar's roster spot, he's not taking over in the rotation. Trevor Bauer will almost certainly be called up for Tuesday's game against Detroit (gulp), at which time someone else will be sent down, probably a reliever.
Salazar is young enough that there's plenty of time for him to develop a more effective arsenal of pitches, and that's easier to do aways from the pressures of MLB, because there's more room for experimentation in the minors.