Ayala has been out of action after picking up an injury against Blackburn Rovers almost two months ago.
Boro have since been in a poor run of form without their leading centre-back, winning only four of nine Championship matches including defeats to Rotherham United and Charlton Athletic.
Commenting on his team-mate’s absence, Kalas says Boro have missed the former Norwich City defender and labels him the best in the Championship when fully fit.
“It’s really difficult to replace Dani Ayala. For me, Dani is the best central defender in the league when he’s playing and not injured. Everyone wants him back so it was a challenge to reach the level that Dani was playing at his best before his injury,” Kalas explained.
Despite Boro’s struggles in Ayala’s absence, their latest last-gasp 1-0 win over Hull City means automatic promotion is still in the club’s hands as we head into the latter stage of the campaign.
Boro currently sit in 3rd spot in the Championship table, one point behind Brighton and Hove Albion having played a game less and with the Seagulls still to visit the Riverside Stadium.
Ayala made a comeback in a recent friendly with Sunderland, and he is now expected to be in contention to return to the starting line-up for the trip to Queens Park Rangers on Friday.