A coming together during the first half of the Champions League quarter-final clash at Old Trafford saw the five-time Ballon d’Or winner come off worse.
There was no malice on the part of the England international, though, and the issue was shrugged off as merely one of those things that happen in the heat of competitive action.
Smalling told BBC Sport of his tangle with Messi: “We spoke afterwards. We had a brief chat and shook hands.
“He knew it was an accident.”
Quizzed further on the aerial challenge which left an Argentine opponent bloodied, Smalling said: “I didn't realise at the time that I'd actually caught him like that.
“[Luis] Suarez [came up to me] after the game as well. We had a good tussle and he just shook my hand afterwards and said 'good luck'.
“It's nice when you can have that battle on the pitch and then there's respect after the game because, ultimately, you are just trying to do your best.”
While Smalling took the fight to Barcelona during a heavyweight European encounter, he was unable to help United keep a clean sheet.
They did, however, overcome a two-goal deficit away from home at Paris Saint-Germain in the last 16 and there remains a belief inside Ole Gunnar Solskjaer’s squad that anything is possible.
Smallling added: “We know it can be done.
“If we can carry on from the second-half performance when we were really on the front foot, brought intensity to the game and caused them a few problems, that's what we need to go there and do.
“We knew that we couldn't go completely gung-ho in that second half because there is another leg.
“We've got to make sure we go there with no regrets and leave everything there to give ourselves the best possible chance.”
Before taking in the must-win trip to Catalunya, top-four hopefuls United will be back in Premier League action on Saturday when they play host to West Ham.