Eickhoff signed with the Rangers and made his professional debut in 2011 with the Rookie Arizona League Rangers of the Rookie Arizona League. He also played for the Class A Short SeasonSpokane Indians of the Northwest League that year. In 2011, Eickhoff's combined record was 1–2, with a 2.37 ERA, and 22 strikeouts, in 19 innings. In 2012, Eickhoff pitched for the Class AHickory Crawdads of the South Atlantic League. He was 13–7 with a 4.69 ERA — his 13 wins were third in the league and matched the 2nd-highest single-season win total in team history. Eickhoff started 2013 with the Class A-Advanced Myrtle Beach Pelicans of the Carolina League, and was promoted to the Double-A Frisco RoughRiders of the Texas League during the season. He had the 3rd-lowest WHIP and the 8th-lowest ERA in the Carolina League, and his 11 wild pitches were 6th in the league. Eickhoff returned to Frisco in 2014, where he was named Pitcher of the Week, on May 5. Eickhoff's 144 strikeouts and 12 wild pitches with Frisco led the Texas League, and his 1.17 WHIP was 4th-best in the league. In 2015, between Frisco, the Round Rock Express, and the Lehigh Valley IronPigs, Eickhoff was 12–5, with a 3.85 ERA, and 126 strikeouts, in 133.1 innings. In 2017, with the Reading Fightin Phils he was 0–1 with a 1.80 ERA in one start. In 2018, with the Clearwater Threshers, Reading, and Lehigh Valley he was 0–1 with a 2.90 in eight starts. In 2019, with Clearwater, Reading, and Lehigh Valley he was 3–2 with a 5.93 ERA in eight starts.
Philadelphia Phillies
On July 31, 2015, the Rangers traded Eickhoff, Nick Williams, Matt Harrison, Jake Thompson, Alec Asher, and Jorge Alfaro to the Philadelphia Phillies for Cole Hamels and Jake Diekman. He made his major league debut for the Phillies on August 21. Eickhoff's 2015 big league pitching totals included a 3-3 win-loss record with a 2.65 ERA, while he struck out 49 batters in 51 innings pitched. In 2016, Eickhoff was 11–14, with a 3.65 ERA, gave up 1.92 walks per 9 innings pitched, had 20 quality starts, struck out 167 batters in 197.1 innings, and had a strikeout-to-walk ratio of 3.976. In 2017, Eickhoff was 4–8, with a 4.71 ERA, and struck out 118 batters, in 128 innings. He held opposing hitters to a.218 batting average with runners in scoring position, the 6th-lowest average among NL pitchers. Eickhoff missed the end of the 2017 season because he was experiencing numbness in his fingers. In 2018, Eickhoff was 0–1, with a 6.75 ERA, and struck out 11 batters in 5.1 innings. In a game on September 28, against Atlanta, he tied a Phillies franchise record shared by Steve Carlton in 1981 and Curt Schilling in 1996, by striking out seven consecutive batters at one point. Eickhoff missed most of the season due to a strained back muscle, followed by a return of the numbness in his fingers, which caused him to be put on the disabled list. He met with a number of specialists during the summer to evaluate nerve damage that was leading to the numbness, twinges, and tingling in the fingers of his right hand, especially when he threw his curveball; their diagnoses ranged from thoracic outlet syndrome to carpal tunnel syndrome, and Eickhoff was given two cortisone shots. In the off-season he signed a one-year contract for $975,000. In 2019 with the Phillies he was 3–4 with one save and a 5.71 ERA, and struck out 51 batters in 58.1 innings. He ended the season on the injured list, first with right biceps tendinitis and then with a blister/laceration on his right middle finger.
