Mason, who has joined the Cubs for the remainder of the season, said Jackson (8-14, 5.34 ERA) ended the year well. The right-hander has added a cutter.
"It gave him another pitch to get outs with when he's behind in the count," Mason said Monday. "You've got to learn to do it there [in the Minors] before you do it here. You don't want to learn to do it here, because that's the hard way."
Jackson, a ninth-round Draft pick in 2008, had a rough May, when he was 0-4 with a 10.38 ERA, but he went 3-3 in his last seven starts and gave up 14 earned runs over 42 2/3 innings in that stretch with 32 strikeouts.
Cabrera finished with 67 strikeouts in 88 2/3 innings over 19 games (17 starts) at Iowa.
"He had the best stuff," Mason said of the right-hander. "What gets him is the inconsistencies. He's young, has tremendous stuff and sometimes he has to back off to throw strikes, which we're encouraging him not to do.
"He wants to compete and wants to get up here so bad he's willing to sacrifice strikes," Mason said. "When he comes [up to the big leagues], he's going to come quick."
Austin Bibens-Dirkx, a 16th-round Draft pick in 2006, finished 4-5 with a 6.07 ERA in 24 games (17 starts) for Iowa, with most of those runs coming in June, when he was 0-2 with a 9.22 ERA in seven games. In his last four games, he gave up six earned runs over 23 2/3 innings and struck out 19.
"He's a grinder, a strike-thrower," Mason said of the right-hander. "He's not going to overpower you. He fits in the Casey Coleman-Randy Wells mold."
Another starter Mason was high on was Rusin, who was 5-2 with a 4.02 ERA in 11 games (nine starts). A fourth-round Draft pick in 2009, he struck out 46 over 62 2/3 innings.
"He's your basic lefty," Mason said. "He can throw five pitches and locate them. He's not going to overpower you, but he does all the little things well. He can bunt, he can field, he can hold runners, he can back-door stuff, front-door stuff. He's a complete pitcher."