Bristol's Richard Hill said of Regan: "I have not asked the physios what his problem is yet but it looked like a neck or head. He's moping about in the dressing room at the moment.
"He would not have come off if he was not 100 per cent so it depends on how he reacts now (on his condition with England). It's not what he would have wanted next week."
Worcester director of rugby Mike Ruddock was trying to keep his team's feet on the ground after they finally broke their Guinness Premiership duck this season by beating a poor Bristol at Sixways.
It has been a frustrating first season in charge of Worcester for the 2005 Welsh Grand Slam-winning coach but claimed his team "went for the jugular" against opposition who looked tired after a hard Heineken Cup campaign.
Ruddock said: "The players are obviously delighted. They are a good bunch and we have been unlucky. We should have won the game up in Newcastle, I felt, where we scored two tries to nil.
"We were powerful today (Saturday) and we looked capable of scoring tries so we are hoping this is a turning point although we are not getting carried away. Leeds could win tomorrow (Sunday) against London Irish and then the heat is back on.
"So we need to use this now to press on. We know we are where we shouldn't be at the moment in the table but this has been a period of transition. There have been a number of new players who have come into the team but we feel that transition has been important.
"Now we are looking at a team which can build and go forward with confidence. There have been a lot of changes and it's taken us a while to bed those guys in."
Hill admitted his side had been second best against a Worcester side who were worthy of the win.
Hill said: "Unfortunately, we didn't turn up today (Saturday) and Worcester did. We were just poor. We cannot produce the big ones week in and week out and we are suffering so badly with injuries at the moment. Today, we picked up three.
"So we are on our backsides at the moment and the three-week break during the 6 Nations cannot come soon enough.
"We had two huge efforts in Paris (against Stade Francais) and against Cardiff Blues (last week) and this one coming after, we weren't on top of our game.
"So much may have been taken out of us in the Heineken Cup and Worcester made far fewer errors than we did today (Saturday). We were littered with errors."