Alejo scored his first goal as a professional on 13 May 2017, netting the first in a 2–0 home win against Rayo Vallecano, but was sent off a minute later.[5] On 14 June, after helping the Madrid side avoid relegation, he signed a four-year contract with SD Eibar.[6]
Alejo made his debut in La Liga on 21 August 2017, replacing goalscorer Charles in a 1–0 away win over Málaga CF.[7] He scored his first goal in the category on 3 December, netting his team's second in a 3–1 home defeat of RCD Espanyol.
On 26 July 2018, Alejo signed a five-year contract with fellow top division side Getafe CF.[8] The following 31 January, after being rarely used, he was loaned to second division side Málaga CF until the end of the season.[9]
On 30 August 2019, Alejo moved to Cádiz CF in the second level, also in a temporary deal, signed until the end of the season.[10][11] The following 22 July, after achieving promotion, the club exercised the obligatory buyout clause and signed him permanently until 2025.[12]
On 31 December 2024, Alejo moved abroad for the first time in his career, after agreeing to a loan deal with Cypriot First Division side APOEL FC.[13]
Personal life
In March 2020, Alejo donated €10,000 to the Hospital Clínico Universitario de Valladolid for the purchase of sanitary supplies needed during the COVID-19 pandemic.[14]