Ole Gunnar Solskjaer’s side travel to Spain on Wednesday ahead of the start of their knockout tie with the La Liga side on Thursday night.
United could be without a number of key players for the match but have at least been boosted by the fact Mata and Telles are now back in contention to feature.
Mata, 32, was given time off following the death of his mother but was seen back at the club’s Carrington training ground on Tuesday afternoon.
The Spaniard has struggled for game time this season and last featured for United in their 3-1 home win against Newcastle back on 21 February.
Telles was also spotted taking part in the first-team training session after he had missed the weekend win over Brighton through illness.
The Brazilian defender has largely played a back-up role to Luke Shaw this season but has started at left-back in the three of the four Europa League fixtures so far.
Marcus Rashford was not spotted by photographers during the session, while fellow forward Anthony Martial is out for the foreseeable future after suffering an injury during the international break.
Teenage striker Anthony Elanga could make his debut after he was added to the club’s B-List on Tuesday afternoon.